A flooded home in Wesport, in 2021. Photo: RNZ / Anan Zaki
A report prepared for the government says 14,500 properties worth $12.9 billion could be expected to experience at least one damaging flood by 2060.
The country should expect the equivalent of last month's Tasman District floods "basically every year", according to the author, with between 300 and 400 homes a year seeing "significant damage" from water reaching at least 30cm above the floorboards.
Read the full report here.
"While you're not going to have Cyclone Gabrielle events [every year], you are going to have a steady stream of people who are going to get hit," said Dr Belinda Storey of consultancy Climate Sigma, which prepared the modelling.
"Those properties are going to be the ones that, if we don't take some action, when they get damaged, they will get damaged again."
"They are going to get hit again, because they are at the bottom of the flood plain," she said.
"My personal view is that this report reinforces that where properties are significantly damaged, we shouldn't be rebuilding them."
Climate Sigma managing director Belinda Storey. Photo: Supplied / Climate Sigma
The estimates are essentially the cost of doing nothing to limit the number of homes at risk from flooding as climate change makes weather events more severe.
The government has been considering ways to limit its own spending on climate and weather-related clean-ups, with government MPs saying taxpayers cannot afford to keep bailing out flood-hit residents from cyclones such as Gabrielle and Hale on a regular basis.
That may mean reconsidering the future of communities in places that "should never have been built on".
However, researchers have warned for years that it will be very difficult for the government to avoid being pressured into buy-outs when faced with the reality of insurance companies retreating, and people losing their homes.
The government is working on a long-awaited climate adaptation bill to set out how communities, individuals and businesses will respond.
An independent expert panel has recommended that homeowners whose houses are flooded or damaged by weather events should not expect buy-outs, and individuals should be responsible for knowing the risks and making their own decisions about whether to move away from high-risk areas.
The panel suggested a transition period of 20 years before people are on their own when it comes to property losses, to provide people with time to make decisions and spread any cost.
The panel did not recommend any coordinated retreat from at-risk areas and critics called the recommendations "unworkable".
A dehumidifier runs inside a home that was drenched by flooding in Tasman last month. Photo: RNZ/Marika Khabazi
Climate Sigma carried out new modelling to go with the report, looking at expected losses to residential homes over the 35 years from 2026 to 2060.
It found that, by the end of the period, 2200 properties worth $1.8 billion would have experienced a flood causing at least 80 percent damage to the house, which it equated to water reaching at least 3 metres above the ground floor.
Five thousand and three hundred homes would have experienced a flood damaging at least half the house, involving water reaching at least 1.2 metres above the ground floor.
Fourteen and a half thousand properties worth $12.9 billion would have experienced at least 20 percent damage to the property, from water reaching at least 30 centimetres above the ground floor.
The biggest number of affected homes - 4500 - was in the Auckland region.
However, when the study looked at the number of houses in line for the worst, 80 percent damage (and flood waters over 3m), the Waikato region had the most homes affected, with 300 compared to Auckland's 200.
Dr Storey said even 10cm-deep flooding can cause serious damage, however only flooding of 30cm or higher was counted in the report.
"Even if you get 10cm of water into your house from floodwater, that can be very disruptive because flood waters almost always include sewage," she said.
About two thirds of the estimated damage was from hazards that already exist, while the final third was because of climate change worsening over those 35 years.
"We already have a significant number of houses at risk," said Storey. "And the probability of flooding inland is approximately doubling in the next thirty years."
Today's risk already includes the impact of around 1°Celsius of global heating, which boosts how much water the atmosphere can hold and worsens heavy rain and flooding.
"It has taken us 170 years to reach the first 1°C and we're going to reach the next 1°C in 30 years, and the next 1°C is going to be about a doubling of the probability [of flooding inland]," said Dr Storey.
Kaipara's Kaihu River, north of Dargaville, flooded during Cyclone Gabrielle. Photo: Kaipara District Council / Supplied
She said the total number of homes affected was smaller than she expected when starting the process, given almost 200,000 New Zealand houses are in flood plains.
"Only a small proportion is likely to be hit at a significant enough level that they would expect the government to intervene," she said.
"A lot of our flood plains are pretty broad and shallow ... [and] a lot of houses might be just high enough with just enough first floor height that they don't get damaged when a flood comes through."
Storey said the figure of 14,500 houses which were expected to be significantly damaged did not mean communities could defend that number of properties to fix the problem. They would need to consider around three times that number, because roughly 45,000 houses in the at-risk category have a 30 percent chance of being hit in next 35 years.
"You can't just defend 14,500 houses and say we've fixed that problem, because you don't know which ones are going to get hit."
Successive governments have considered introducing measures, including making it harder to build in flood zones, improving risk information, and setting out what happens when an uninsured property floods - or when it's time to abandon land.
But no legislation has yet made it to Parliament. A new law has been promised by the end of this year.
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