Source: NZ Department of Conservation
Date: 17 November 2025
The Pou Taiao of Ngāti Koata, Neil Deans, says Lake Moawhitu was once an important kainga and mahinga kai site and used as a tuna (eel) fishery by generations of tangata whenua.
But after a century of farming in the Lake Moawhitu catchment, it now suffers from high nutrient levels, poor water quality and algal blooms.
“Restoring Lake Moawhitu and its surrounding catchment and downstream wetland is critical for Ngāti Koata,” Neil says.
DOC Senior Science Advisor Katie Collins says historic use of fertiliser, as well as erosion following deforestation, has resulted in a legacy of high phosphorus levels in lake sediments, which fuels growth of algae.
“Lake Moawhitu is ‘super-trophic’, meaning high levels of nutrients and low water clarity. Algal blooms can occur over summer, limiting native aquatic plant and animal growth, so the lake is in a perpetual state of poor health.”
An operation to lower phosphorus levels and restore the lake took place at the end September, involving application by helicopter of alum (aluminium sulphate) over deeper parts of the lake.
Areas of Lake Moawhitu were treated with alum, which binds with phosphorus and deactivates it before settling to the lakebed, creating an inactive layer that won’t fuel algal growth.
Project Manager of the Moawhitu restoration work, Dan Moore, says better water quality will help native aquatic plants grow, improving long term lake health and opportunities for wildlife and fish to thrive in the lake.
“The project team worked with researchers from the Cawthron Institute, NIWA and the University of Waikato to consider phosphorus management options and saw evidence that ‘alum’ operations work well to bind phosphorus and control algal blooms.
“Ongoing monitoring will determine the alum application’s success this summer, when algal blooms often occur. We hope to see a major improvement in water clarity.”
As well as alum dosing, Ngāti Koata, with DOC, Marlborough District Council, private funders and many supporters, have planted nearly 150,000 trees in the lake’s catchment over more than a decade.
Katie says communities need to work together to make a difference to help lakes with poor water quality like Moawhitu.
“Think about how you can help to our waterways when you’re out naturing – that can be anything from joining your local stream care group to checking, cleaning and drying your gear before you head to the water.”
For Ngāti Koata, the ultimate objective is getting healthy catches of tuna from the lake and renewing the relationship of rangatahi (young people) as future lake custodians.
Background information
Alum (aluminium sulphate) is a type of salt derived from metals. It’s used for lake treatments internationally and around the Rotorua lakes in New Zealand. For this operation, we ensured it was applied in a way to minimise any impact on fish.
Lake Moawhitu, the adjacent wetland and surrounding catchment were purchased in 2005 and added to the adjacent reserve and recognised in the Ngāti Koata Deed of Settlement. Since then, restoration work has including destocking, replanting and raising water levels in the adjacent wetland.
Lake Moawhitu is part of DOC’s Arawai Kākāriki wetland restoration programme. Work to improve the lake involves DOC, Ngāti Koata and Marlborough District Council with funding from multiple sources.
Visit He reo nō te puehu – Discover Lake Moawhitu, hosted by Our Lakes Our Future, for the lake’s history and a vision for the future.
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