High seas lash cars driving on the St Clair Esplanade. Photo: Gerard O'Brien

More than $400million worth of roads and other assets owned by the Dunedin City Council are under threat from predicted sea level rise, underscoring the need for government help, Mayor Dave Cull says.

Dave Cull

The figures for Dunedin were released as a major report by Local Government New Zealand (LGNZ) outlined the expected costs from sea level rise for councils across the country.

The report estimated $5.1billion of council assets across the country - including roads, pipes, buildings and other facilities - were at risk from 1m of sea level rise by 2100.

The estimated cost jumped to nearly $8billion for 1.5m of sea level rise, and to $13.3billion for 3m of sea level rise.

Nasa scientists have predicted sea levels will rise between 30cm and 1.3m by 2100, while Niwa has been more conservative, estimating an increase of between 40cm and 1m.

But even under a 1m sea level rise scenario, $446million worth of infrastructure would be left exposed in Otago, the report said.

The bulk of Otago's infrastructure under threat belonged to the Dunedin City Council, which owned $415million of the exposed assets, council staff confirmed yesterday.

That included pipes, roads and buildings, but also green spaces and landfills.

Mr Cull - who is also LGNZ president - said the figures underscored how far behind the country was in identifying, and responding to, the challenges posed by sea level rise.

The local government sector had been "leading the way", but the Government needed to act, as the costs would only continue to climb, he said.

"The danger is the longer we go not knowing how we're going to spread the financial burden, the longer councils are going to have to put it off, because they just can't find ways of funding it, and the bill gets bigger.

"Ultimately, government has to come to the party."

The report identified Otago as a "priority region" for threatened water-supply, stormwater and wastewater assets, including pumping stations and hundreds of kilometres of pipes.

But the report also warned Otago had the highest value of roads under threat from 0.5m or 1m of sea level rise, coupled with a significantly smaller population of ratepayers to pay the costs of adaptation.

The region's "critical" coastal tourism infrastructure would also be impacted, "potentially affecting local economic productivity and business development".

As a result, LGNZ was calling for government action, including the creation of a new local government risk agency and a national climate change adaptation fund.

Councils would also need to adjust their planning approaches, "avoiding areas exposed to sea level rise".

A national management retreat programme would also be needed for buildings and other facilities, and options for under-threat areas could include "demolition, adaptation or retreat".

In South Dunedin, other options - such as new drainage infrastructure - could forestall any "drastic" decisions, Mr Cull said.

Simply abandoning the suburb was "not viable, and it wouldn't be fair, and it wouldn't be affordable either".