Inland flooding costs could rise by up to 53 percent by 2075

Damage and repair costs from inland flooding are expected to rise by up to 53 percent by 2075, with some regions worse hit than others, a new report shows.

The cost of coastal flooding is currently lower, but is expected to rise faster - more than doubling under higher-emissions scenarios.

Following a five-year project that last year mapped flood risk across the country, the Earth Sciences study on behalf of the Infrastructure Commission used valuation data from the commission to model the increased costs of flooding as a result of climate change.

"By 2075, losses increase across all emissions scenarios," the report said.

It showed that under 2025 conditions, inland flooding costs were estimated at an average $471.9 million a year - which included private buildings ($190.8m), water infrastructure ($68.4m), local roads ($46.9m) and central roads ($25.3m).

That cost would rise by 45 percent - to about $682m a year - by 2075 under a medium greenhouse gas emissions scenario; by 50 percent to $707.2m under "high" emissions; and a 53 percent increase to $721.3m under a "very high" scenario.

Damaging floods do not hit every city or region every year, and the report warned this meant losses in a given year "may deviate substantially" from the annual average.

Coastal flooding losses - driven by sea level rise - would take longer to arrive but at an accelerating rate - going from $242.7m a year to $461.7m, $503.6m and $542.7m under the different emissions scenarios.

Infrastructure Commission economics and research director Graham Campbell welcomed the report, saying decisions made today would 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."

The effects would not be felt evenly across the country.

In 2025, Auckland topped the tables for highest average annual losses at $101.7m, followed by Christchurch ($65.1m), Palmerston North ($39.5m), Napier ($34.3m) and Waikato ($26m).

"Palmerston North City stands out for its relatively high losses in education and government infrastructure, suggesting a concentration of public assets in hazard areas," the report said.

However, by 2075 under a high-emissions scenario, the areas expecting the biggest increases in losses were Hauraki (244 percent increase compared to 2025), Nelson City (166 percent), Buller (117 percent), Whakatāne (116 percent) and Napier (113 percent).

Broken down by type of infrastructure, rail infrastructure was most at risk.

Annual rail losses from inland flooding in 2025 amounted to 0.31 percent of total network value, rising to 0.55 percent a year by 2075 under the very-high emissions scenario.

This was followed by water infrastructure (from 0.21 percent to 0.36 percent), electricity networks (0.20 percent to 0.34 percent) and local roads (0.16 to 0.27).

The emissions scenarios used were Shared Socioeconomic Pathways or SSPs, an internationally recognised metric. The medium scenario estimates global temperature rise of about 2.7C above pre-industrial levels by 2100, high represents a 3.6C increase, and very high a 4.4C increase.

To compare apples with apples, the modelling assumed assets would remain as they are, and did not account for potential future investments.