These Waikato hydro lakes are supposed to be safe to swim in, but a toxic algae problem is getting worse

Source: Radio New Zealand

In a finger-like branch of Lake Ohakuri the water is as green as the grassy paddocks surrounding it.

It’s thick with algae. The microscopic plant-like organisms lack leaves or roots, but possess a prodigious ability to feast on nutrients and with the help of sunlight, multiply exponentially, turning clear water into a murky, and sometimes toxic, spinach soup.

As unappealing as the water looks, today is a “good” day. On bad days, the algae clump together like old friends embracing, creating snot-like mats of slime. On really bad days, there’s a stench of rot and death.

Despite a legislated vision for the water to be safe to swim in and talks starting more than a decade ago to reduce nutrients in the river, summer algal blooms plague Lake Ohakuri and other hydro lakes along the Waikato River.

Swimming in this water is dicing with illness.

Peter Withers co-owns a five acre block on the Whirinaki Arm of the lake. It was intended to be a summer bolthole, but the water conditions here are frequently poor.

A pontoon moored off the shore bobs hopefully in the green water. It was built to support summers of fun, but there are days when the pontoon stays dry. Withers swims here sometimes, but not on the days when it’s radioactive green, or when the snot clumps have formed.

Yes, sometimes the water gives him a sore throat, he says, but he likes to think he judges the conditions well enough to not get seriously ill. He’s careful about not putting his head under.

He’s far more cautious when it comes to his children; they are often banned from entering the river completely. Algae can produce toxins which attack the liver.

Leanne Archer lives further around the lake. She describes the water as often neon-green, and sometimes smelling of rot. She would like to spend summer enjoying the lake but the potentially toxic algal blooms means she keeps her distance. Her dog Misty loves to swim, but has to be kept inside. The few times she escaped and played in the water she became sick, “vomiting and vomiting,” says Archer.

Katrin Halbert is another Lake Ohakuri local with a dog. On its worst days she says the Whirinaki Arm of the lake is fluorescent green. “You know, The Hulk, The Incredible Hulk, when it turns green, it’s like that.” Last year one of her dogs drank from the lake and within half an hour started vomiting. The days of walking her dogs in the reserve are over, she says.

For dogs, cyanobacteria can be lethal even when the toxins are at a level below what would trigger a public health warning.

There’s a bevy of organisations playing a part in the Waikato River.

Mercury Energy runs the hydro electric power plants that slow the river’s flow. The Waikato Regional Council grants resource consents and sets the rules for land and nutrient use in the catchment. The Waikato River Authority is responsible for the legislated vision of a river that is safe to swim in. Joining them are property owners dotted along the river including farms, forests and industry.

All the big players are aware of the algal blooms, but each says it’s following the rules. An algal bloom working group they belong to has met for the past three years, but frustrated locals aren’t seeing any concrete action. They have taken to documenting lake conditions on a public Facebook page. It’s littered with images of sunny days and lurid green water.

The toxic problem

Before the Waikato River was dammed, a drop of water could travel its 425 kilometre length in seven days. Today that same drop, beginning in the clear waters of Lake Taupō, is slowed by a chain of hydro lakes. By the time it reaches Port Waikato it is murky brown, and the journey has stretched into weeks.

The eight hydro lakes along the river act as batteries for Mercury Energy, holding water to generate power and producing around 10 percent of New Zealand’s electricity.

Some locals think the dams and how water levels are managed contribute to conditions conducive to algal blooms. Others point to weed spraying and say the nutrients released as the weeds in the lake die, turbo charge algae growth.

Some blame a continual flow of nutrients like nitrogen and phosphorus, seeping in from the farms, horticulture and industry in the area. Climate change gets a mention too, warm weather and tempestuous summer storms can sweep even more nutrients from land into the river system, providing a banquet for algae growth.

Algae aren’t necessarily bad; they’re an important part of freshwater ecosystems, providing food for the invertebrates that fish feed on. But sunshine, warm temperatures and slow water flow, combined with an abundant supply of nutrients can lead to population booms known as blooms.

The boom turns to a bust when conditions change. This might be a drop in temperature or when the algae have consumed all the nutrients in the water. As the algae decomposes its cells collapse and cyanotoxins can be released.

Not all types of algae produce toxins, but the ones that do can make the water poisonous, triggering asthma and hayfever, skin rashes, stomach upsets, tingling around the mouth, headaches, breathing difficulties and visual problems.

Worryingly, data from the Waikato Regional Council shows the proportion of cyanobacteria (the type of algae that is toxic) in the hydro lakes it monitors is slowly increasing during summer months and becoming the predominant algae.

It’s the amount of cyanobacteria present which can trigger a public health alert.

At Lake Arapuni, one of the last hydro lakes on the river, Ryan Fynn keeps his dog in his ute when he meets us. It’s never allowed near the water when it looks green.

His first memory of the lake was as a four-year-old, standing on the front of his uncle’s water skis. Back then they piped the water from the lake straight to their house as drinking water. Now, it’s a different story.

Some days, when the wind blows in the right direction it drags a scent from the lake, which he describes as “sewagey”, into his windows. This summer the algae was “like a big, thick mat” his boat struggled to get through.

He’s battled to get satisfactory answers from the Waikato Regional Council. “They just fob everyone off.”

Fynn thinks part of the problem stems from how long water is held in the lakes by Mercury Energy, the company which runs the hydro dams. The longer it’s held, the longer the algae has a chance to grow, he suggests.

When RNZ meets Fynn on 3 February, the water looks clear, but a sign warns there are high levels of cyanobacteria.

It’s the same sign that was seen by RNZ on a sunny Sunday in January. That day, the water also looked clear and children splashed happily next to the sign.

Keeping people safe

Data doesn’t lie, but the data Waikato Regional Council uses to assess whether the water is poisonous doesn’t necessarily give a full picture of what’s going on day-to-day.

Toxic conditions could occur more often than what has been recorded without triggering public health warnings and sometimes health warnings remain in place when water conditions have improved.

Only four lakes (Ohakuri, Maraetai, Karāpiro and more recently Arapuni) are tested for cyanobacteria once a month between November and April. Tests are done on a Monday and results come back Wednesday. If toxin levels exceed safe recreational thresholds signs are put up, and a public health warning is given. Testing is supposed to increase to at least every seven days.

Weekly sampling from December to March is done at the same four lakes, but this measures a pigment only found in cyanobacteria, not the toxin itself. The council is yet to work out how to use this result to estimate whether toxin levels breach guidelines.

Separate state of the environment monitoring is done at six points along the river. These monthly samples don’t test for cyanobacteria, but do test for nutrient levels such as nitrogen and phosphorus, which are nutrients algae feed on.

Rob Dexter is part of Let’s Be Clear, a charity aiming to help improve water quality. He has reservations about what can be learnt from snapshots in time like this.

The way he describes it, monthly cyanobacteria testing is like testing a person with diabetes once a month and saying those readings are indicative of their daily blood sugar levels.

When it comes to human health, he’s doubtful the current system of infrequent testing with a 48 hour turnaround time for results – then another lag before warning signs are erected – is adequate. Blooms may have ceased before the signs go up, or have moved further down river.

He wants continuous monitoring, with real-time data, including webcam images, shared openly.

While technology for continuously monitoring cyanobacteria doesn’t exist in a cost-effective way, he argues “surrogates” can be used. Combined with weather and flow, the likelihood of a bloom could be modelled. When cloud cover isn’t an issue satellite images can also show blooms from space, which he says could be used in conjunction with webcam images.

Dexter has installed real time monitoring systems on private properties along the river at his own cost. They have shown spikes of nitrogen entering the water after major storms, such as Cyclone Gabrielle. An algal bloom occurred in the weeks after the storm.

He believes resource consents should be evaluated against Te Ture Whaimana o Te Awa o Waikato, the legislated vision and strategy for the Waikato River, established as part of a Treaty settlement. The vision calls for water that is safe to swim in and take food from along its entire length.

“I believe almost every decision on the Waikato River is being made based on inadequate data sets that weren’t designed for the intent that it’s been used for.”

Institutions are following the rules, but are the rules fit for purpose?

Mercury Energy, the Waikato Regional Council and the Waikato River Authority have defined roles and rules, and while organisations say they are meeting them, this is not stopping the ongoing appearance of blooms.

Emily Collis is the operations lead at Mercury Energy. The company is aware of the algal blooms that plague the hydro lakes but she doesn’t accept that the power company is part of the problem.

“At this stage, there’s no evidence to suggest that the way we’re managing the river is contributing.”

The company repeatedly told RNZ it operates within the rules of the resource consents that dictate the amount of flow and the lake levels. Its contractual agreement to supply electricity also plays a part in the way it manages the lakes.

Collis has heard the suggestions that flushing the lakes will clear blooms, but doesn’t feel that would solve the problem.

“We haven’t done any particular studies that we would then go and share or publish or anything like that. But because we are active on the river every day, we do have observations that tend us to believe that even if we were in high flow conditions, a lot of the algae blooms will not move simply because of their location in those slower moving areas,” she says.

The spraying of diquat to kill weeds that can clog the dam turbines is a sore point for locals, who say they notice blooms after spraying.

Mercury says it has used diquat for 17 years and algal blooms have not always occurred after its use.

Research completed by NIWA, (now Earth Sciences New Zealand) showed weeds release a large amount of nutrients when they die. It suggested more research was needed to understand the effects in a lake system. When asked if Mercury would fund research into this Collis indicated the company was “open to opportunities”.

Is she at all concerned the company will lose social license in the communities it operates in? Mercury has a hydro stakeholder manager who is often in the community helping people understand Mercury’s role in the river, she says. The company also tries to support communities with funds and sponsorships of the Waikato River Trail.

Mercury points to Waikato Regional Council as the agency responsible for the management of the river’s water quality.

The council’s science manager Mike Scarsbrook says the organisation is worried about the changes seen in the river. He says the blooms are becoming more frequent and more severe.

“It’s not a good space for us to be in. We are working towards improving the health and wellbeing of the river, but we’re certainly seeing worrying signs.”

Climate change plays a role, but the blooms also need nutrients to grow, he says. This can come from nitrogen and phosphate washing in from farms, or other enterprises dotted along the river.

The council is proposing significant changes to how the land surrounding the river is used, which include measures to reduce the nutrients entering the water. Proposed plan change 1 is the council’s answer to meeting Te Ture Whaimana o Te Awa o Waikato’s legislated vision of water that is safe for swimming and food gathering.

The plan change was first notified in 2016 and applies to approximately 10,000 properties and covers more than a million hectares within the Waikato and Waipā river catchments. Over a decade on, it’s still not enacted. Currently it sits with the Environment Court, caught up in a slew of appeals, although interim decisions made by the court look promising for the plan’s future.

If it does come into effect, change won’t happen overnight. Improvement has been given an 80-year runway, meaning the reduction in nutrient levels in the river isn’t expected to be reached until 2096.

Not everybody thinks the reductions will have an effect. Former Fish and Game water scientist Adam Canning previously told RNZ the nutrient reduction targets in the plan were watered down to a level he described as maintaining the “status quo”.

“It’s pathetic. And we don’t have to achieve it for 80 years. 2096. I’ll be dead.”

Canning doesn’t believe the reductions will ensure the water is safe for swimming and food gathering, achieving the legislated vision of Te Ture Whaimana o Te Awa o Waikato.

Chief executive of the Waikato River Authority Antoine Coffin is the man in charge of Te Ture Whaimana o Te Awa o Waikato. The Authority has iwi and Crown representatives on its board and distributes funding for restoration projects. It sets the vision for the health of the Waikato River.

“We’ve been set up as a voice for the river,” says Coffin.

But having a voice isn’t the same as having teeth. The authority’s vision sits above other legislation, such as the Resource Management Act, but the authority doesn’t have enforcement powers. It can’t compel Mercury Energy to increase water flow to see if that affects blooms, demand the council’s new plan be put in place before the Environment Court process runs its course, or tell farmers to cut back on fertiliser.

Asked whether the Waikato River Authority was taking a leadership role on algal blooms, Coffin does not directly answer, but says long-term solutions, such as the council’s new plan are important. He doesn’t think the plan will fix everything, “but it’s a good start”.

Continuing algal blooms, “would be an antithesis to the vision and a strategy for the river,” he says.

He’s part of the Upper Waikato Algal Blooms Working Group. Other members include Waikato Regional Council, Raukawa, Ngāti Tahu – Ngāti Whaoa Rūnanga Trust, Te Arawa River Iwi Trust, Taupō District Council, Let’s Be Clear Trust, Mercury Energy, Tūwharetoa Māori Trust Board and Dairy NZ.

The group met twice in 2024 and once in 2025 and 2026. Its aim is to reach an understanding of the drivers of the algal blooms and, according to a memo from a September 2025 meeting, “to advance a Response Plan for managing the impacts of harmful algal blooms in the Upper Waikato – within the context of existing interventions to manage water quality, such as the Waikato Regional Council Proposed Plan Change 1, and Te Ture Whaimana o awa o Waikato.”

One of the things the group is working on is communication. “Together, we are also working to improve public understanding of risks, in particular, what specific language will help people to heed those risks – we understand that many people still use the water even when health warnings are in place,” a council statement said.

Coffin is confident the working group is more than a box-ticking exercise but says getting alignment between the various groups, each with their own mandate, is a “conundrum”.

Community takes control

For locals, the stance of the big players, ranging from denying any impact, to having their hands tied by rules, offers little comfort. They’re yet to see any improvement as a result of the working group.

Hope Woodward is a councillor for the Mangakino-Pouakani ward of the Taupō District Council.

“Heartbreaking,” is a term she uses repeatedly about the water condition, and she’s agitating for action.

She’s attended working group meetings and suggested a survey be conducted to understand how the water quality affects the community. The Waikato Regional Council agreed to her idea, she says, but when she asked when it would be done the council’s response was, “in the next fiscal year, funding dependent”.

She set up a survey herself using an online tool that cost $50. Eighty-one of the almost 100 people who completed it said poor water quality stopped activities. At least 41 percent of those who responded to the survey said they or their pets had been sick after being in contact with the water.

Almost half of the comments touched on perceived inaction and fragmented responsibility.

“There seems to be nothing happening, we complained two years ago and nothing changed,” one respondent wrote.

“Somebody needs to take accountability and fix it,” another commented.

Woodward says the issue affects public health, livelihoods, tourism, property values and wellbeing.

“I think the responsible entities just need to stop having all these discussions that have no resolutions whatsoever. It just seems that nobody wants to take any accountability for what’s gone on.”

She wants farming consents to be checked, and she’s keen for more research to be done into the effect of Mercury spraying weeds in the lakes. She can’t identify a single thing being done now that will stop another summer of blooms.

“There’s a lot of data gathering and discussion happening, which is important, however we now need to see that translate into real, on the ground action.”

The working group’s eventual response plan will only satisfy the community if it’s backed by accountability and delivery, she says.

At Lake Maraetai in Mangakino, Karl Hitchcock says the blooms have gotten worse since he bought his property.

“We’ve got pictures of the lake. It’s just glowing in the dark. It looks radioactive.”

It makes wakeboarding and swimming unsafe and there’s a flow-on effect for the community. Businesses which rely on the weekend recreational visitors are struggling, people just don’t bother coming anymore. There’s only one upside, he jokes. “It’s good for the swimming pool, because all the locals will swim in the swimming pool, right?”

He’s been one of the key voices on the community Facebook page encouraging people to send in photos of water conditions and looking for opportunities to improve the situation.

Hitchcock has looked into whether a fund administered by the Waikato Regional Council might be applied to pay for webcams at the lakes, but was told it probably wasn’t the right fit for that particular fund. He had hoped giving the ability for people to see whether the water was pea soup green, or covered in scum might prevent wasted trips to the lake.

The council told RNZ webcams are one of the options the working group is considering, along with drone footage, community reporting via photo and satellite imagery.

He’s now planning to apply for funding from Mercury Energy to try an ultrasonic treatment for the lakes where blooms occur. This would consist of a solar-powered floating unit that emits sound waves that kill algae.

Without some concrete action he thinks next summer, “will only be worse”.

The river is an asset to farmers, recreational users and the country’s power generation capability. “Obviously, we’re generating a lot of power from the dam, and everybody wants cleaner, cheaper power, but we all just need to get together and fix it.”

He’s another fan of more data being gathered to help drive decisions about how to improve the water quality.

“When the water comes out of the Taupō gates, it’s crystal, it’s pristine, it’s so good up there. But then when it comes down here, it’s green, and it doesn’t need to be.”

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– Published by EveningReport.nz and AsiaPacificReport.nz, see: MIL OSI in partnership with Radio New Zealand