Covering climate now series: How rising seas could cost NZ billions

South Dunedin resident Eleanor Doig, with her century-old home that's already been badly hit by flooding. Photo / Peter McIntosh

No one can tell Eleanor Doig that something strange isn't going on in South Dunedin.

Eight years ago, after a heavy rain, she could quite comfortably walk to the washing line out the back of her century-old Musselburgh cottage.

The ground might have been a little soggy. A few years after that, she needed gumboots. Then, the water reached halfway up them.

She happens to live in what's a working case study for researchers studying the impacts of sea level rise.

South Dunedin - home to about 10,000 residents, 12 schools and six rest homes - straddles a spread of low-lying flats between Otago Harbour and the Pacific Ocean.

Back in June 2015, when a monster deluge unloaded 142mm of rain in just 24 hours, hundreds of homes – Doig's among them - were swamped.

In the aftermath, local drainage experts pointed to a faulty stormwater system. Others argue the big flood would have happened anyway.

In any case, the coming impacts of climate change don't fit well with South Dunedin's ageing infrastructure - a feature of so many other New Zealand towns and cities.

Dunedin Mayor Dave Cull has likened the situation in his city to a slow-moving earthquake, and probably for good reason.

Even just 20cm to 40cm of sea-level rise will mean the suburb's water table will rise, exposing it to flooding after heavy rain that could damage roads, pipes, cables, buildings and parks.

Eleanor Doig says the soil in her South Dunedin section has become noticeably soggier after heavy rains. Photo / Peter McIntosh

"There are a lot of sections on the flat we're on where people have had to put in sumps – there's been low-lying water whenever there is rain, so the water table seems to have been getting higher and higher," Doig said.

"The scientists are doing research around how much of this is groundwater – and how much is saline, or coming from a higher sea level."

"Putting bigger drains in won't solve the problem, as the water flow isn't fast enough, you get silt, and they clog up."

Looking around her neighbourhood, she worries. Sixty per cent of Dunedin South residents are renters. Where will they go if rising seas force them out? She sees the problem as not just a financial and political one, but one of social justice.

"We need to get everyone – the council, the community, everybody – engaged in finding solutions."

Councils elsewhere aren't under any illusions about the unfolding planning and regulatory nightmares they and their ratepayers face.

Recent modelling shows about 72,000 New Zealanders are already exposed to extreme coastal flooding, along with about 50,000 buildings worth $12.5 billion.

This risk increases markedly with sea level rise, particularly during the first metre of rise we can expect this century.

There is near certainty the sea will rise 20cm to 30cm by 2040 and, by the end of the century, depending on whether global greenhouse gas emissions were reduced, it could rise by between 50cm and 1.1m.

That's an extra 116,000 people exposed to extreme coastal storm flooding.

On top of that, some 700,000 people and 411,516 buildings worth $135b would be exposed to river flooding that could come amid extreme weather.

In our largest city, more than 43,000 Aucklanders are directly threatened by rising seas - up from 34,700 people in 2001.

Up to $14b worth of ratepayer-owned infrastructure is at risk of sea level rise. Photo / NZ Herald

In the eventual firing line of rising water will be Auckland Airport, Middlemore Hospital, much of the CBD area - and even the local Civil Defence offices.

Local Government New Zealand has urged ministers to launch a new state adaptation fund and risk agency, so they won't face the threat without the resources and expertise they desperately need.

Its own research shows as much as $14b of ratepayer-owned infrastructure is at risk of sea level rise - including about $2.7b of roading, water, and building infrastructure threatened by as little as half a metre of it.

Again, the cost ramps up sharply at each increment of sea level rise, with the data showing that $5.1b worth is at risk at 1m of sea level rise, $7.1b at risk at 1.5m and $14.1b at risk at 3m.

In seaside spots like Matata, in the eastern Bay of Plenty, some councils are already getting a lesson in the hard realities of coastal retreat.

The insurance industry has also warned of increasing levels of excess for vulnerable properties over time - if insurers opt to take on the risks at all.

The Government has responded, partly, by setting up a new national risk assessment system.

Whatever the future looks like, Doig is certain her suburb wouldn't be the same for much longer. A response to the threat has to come - and soon.

"It's a very real issue - and pretending that it's not is stupid."

Tomorrow: New Zealand's rising fire risk

This story is part of the Herald's contribution to Covering Climate Now, an international campaign by more than 170 media organisations to draw attention to the issue of climate change ahead of a United Nations summit on September 23. To read more of our coverage go to nzherald.co.nz/climate