5 Jul 2023

More than 400,000 residential buildings in flood prone areas

3:00 pm on 5 July 2023
A house on the banks of Nelson's flooded Maitai River on 18 August, 2022.

A house on the banks of Nelson's flooded Maitai River on 18 August, 2022. Photo: RNZ / Angus Dreaver

Twelve percent of New Zealand's housing value is in a flood hazard area, new research shows.

The report by NIWA and the University of Auckland found that 441,384 residential buildings were at risk of flooding, with an estimated replacement value of $218 billion.

The figures comprise 282,395 houses valued at $213 billion and 158,989 appurtenant buildings (like shed and sleepouts) valued at $5 billion.

Most were in urban areas, with Auckland and Canterbury accounting for around half of the buildings.

The Wellington, Waikato and Bay of Plenty regions also had many buildings affected.

NIWA hazard analyst Ryan Paulik said while New Zealand had a lot of residential buildings that could be exposed to fluvial flood hazards, a large number of those flood hazards were protected to some degree by stop banks and other flood mitigation structures.

"So it's really important to understand what the frequency of potential flood exposure could be," he said.

"Because a number of buildings that are identified as exposed to flooding might only be exposed to flooding under really rare circumstances."

A limitation of the study, Paulik said, was that it relied on existing flood maps.

This meant rare flood events with high intensity and magnitude such as Cyclone Gabrielle may not be factored.

"These rare events may not be represented sufficiently through flood maps that are available publicly," he said.

"So with these rare events, there's an opportunity to remap the flood plains and their extent, and then go and collect information about residential building exposure within these areas."

Still building in flood risk areas

The research found that building construction in flood hazard areas peaked between 1960 and 1980.

But although construction has slowed in subsequent decades, the total floor area and replacement value has continued to increase.

And while construction may have slowed, plenty of houses are still being built in flood risk areas.

In March, RNZ reported that council consent staff were being pressured to give the go-ahead to developments in flood-risk areas in the greater Wellington region.

While in April, it was reported that more than 15 percent of the state housing portfolio was on flood-prone land, and Kāinga Ora planned to continue putting new builds on land it knows will flood in the future.

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