Source: New Zealand Infrastructure Commission
New research by Earth Sciences New Zealand, commissioned by the New Zealand Infrastructure Commission, Te Waihanga, explores how flood damage to infrastructure could change under different climate scenarios. It shows that, across the scenarios modelled, flood damage is expected to increase over the coming decades, with the impacts felt unevenly across the country.
The Commission’s cover note, ‘When waters rise: Climate change, regional risk and infrastructure investment’, summarises the research and highlights that flood-related financial losses are expected to increase significantly under these scenarios and, in most regions, exceed those from any other natural hazard over the coming decades.
“Infrastructure decisions made today will shape New Zealand for decades. While we cannot predict exactly how the future will unfold, scenarios help us consider a range of plausible futures, understand the implications of different choices, and build infrastructure that is more resilient to uncertainty,” says Graham Campbell, Director – Economics and Research, New Zealand Infrastructure Commission.
“Financial losses to infrastructure rise significantly under any modelled climate scenario. Under a scenario with high greenhouse gas emissions, the annual cost of inland flood damage to New Zealand’s infrastructure – our roads, our powerlines and other critical services – is estimated to increase from $300 million per year in 2025 to around $465 million in 2075. Coastal flooding damages are expected to almost double: from $165 million per year in 2025 to around $325 million in 2075.”
Dr Nick Horspool, Senior Natural Hazard Risk Scientist, Earth Sciences New Zealand, says the flooding will not be experienced evenly across the country.
“Some regions, like Hawke’s Bay and Manawatū-Whanganui, face high expected losses relative to the overall size of their infrastructure networks – around double the national average. Regions such as Auckland, Wellington, and Taranaki tend to face lower losses proportionate to infrastructure exposed.
“The research shows that most of the increase in inland flooding losses under a high greenhouse gas emission scenario occurs over the coming decades, while coastal flooding losses grow gradually but at an accelerating rate over time,” Horspool says.
“We also modelled future losses to private buildings like homes and businesses, helping Te Waihanga assess the need for future flood protection investments. Like infrastructure, this modelling also finds that annual flood damage to private buildings is forecast to grow significantly over the next 50 years. This is particularly true in places like the West Coast, Hawke’s Bay, and the Bay of Plenty.”
Campbell says that making the right decisions requires knowing the size of the risk and the cost of inaction.
“If we don’t size the risk, investment decisions risk being either too cautious or too costly. The research shows that flood losses are real and growing, but also that they can be best managed with strong asset management practices, using insurance costs to help assess options, and prioritising cost-effective flood protection infrastructure. The right response will differ by region, sector, and hazard,” Campbell says.
This work provides a high-level understanding of the scale of potential damage to our infrastructure networks from natural hazards. It provides an evidence base to support one of the National Infrastructure Plan’s top ten priorities – identifying cost-effective flood risk infrastructure.
Notes:
- The research, research note and associated data book will be available on the Te Waihanga website from 5.00am 9 July.
- Average annual losses (AAL) are the expected financial cost of damage from natural hazards each year, averaged over many years. In some years, damages will be much higher than the AAL, while in other years it will be lower.
- Under the SSP3-7.0 climate scenario, average annual losses for infrastructure from inland flooding are expected to grow from about $300 million per year in 2025 to almost $465 million by 2075. Coastal flooding losses are expected to grow from $165 million per year in 2025 to $325 million in 2075. These losses reflect the financial book value losses to infrastructure, not the full replacement value.
- Key regions with highest increases in flood losses to infrastructure relative to their network size under the SSP3-7.0 climate scenario:
- Hawke’s Bay: From $43 million in 2025 to around $89 million by 2075
- West Coast: From $12 million in 2025 to around $24 million in 2075
- Nelson-Tasman: From $13 million in 2025 to around $25 million in 2075
- Manawatū-Whanganui: From $53 million in 2025 to around $85 million in 2075
- Bay of Plenty: From $45 million in 2025 to around $78 million in 2075.
- In 2025 and in total dollar terms, Auckland is the region with the highest flood losses at around $66 million. However, by 2075, climate change sees Auckland flood losses (around $97 million) overtaken by Canterbury (around $113 million) and Waikato (around $107 million) in the SSP3-7.0 scenario.
- Average annual flood (inland and coastal) losses to private buildings are expected to grow from $270 million per year in 2025 to around $420 million per year by 2075 in the SSP3-7.0 scenario. These losses reflect the replacement value of private buildings.
- Regions with highest private building losses as a share of asset value include West Coast, Hawke’s Bay, Bay of Plenty, and Nelson-Tasman.
- These flooding estimates do not include the losses related to rainfall-induced landslides or predict future growth or development patterns.