Much of South Dunedin housing will be at sea level or below in just 17 years, according to an Otago academic.
Professor Jim Flynn of Otago University said sea level rise had sped up in recent years, increasing the threat to low-lying housing.
Another expert, however, challenged the credibility of the data underlying that prediction.
This street in South Dunedin was among those hit by flooding in June 2015. Photo: RNZ / Ian Telfer
A serious flood in June 2015 damaged about 1250 properties, and a report released in November by Parliamentary Commissioner for the Environment Jan Wright made it clear the problem of rising sea levels was just beginning.
Emeritus Professor Jim Flynn Photo: University of Otago
Dunedin City Council has been relying on projections that the sea level will rise by 30cm over half a century.
Prof Flynn, Emeritus Professor in Politics at Otago University, said in a presentation to the council that since the Greenland ice cap starting melting three years ago, the rise had sped up dramatically.
He calculated 1900 houses in South Dunedin within 25cm of the current water level would be flooded by 2034.
That meant South Dunedin would not be able to be defended for long and plans must be made to move residents to higher ground.
About 2700 homes in South Dunedin lie less than 50cm above sea level.
Dunedin mayor Dave Cull said councillors were a little stunned by Prof Flynn's presentation, and it had heightened their sense of urgency to respond to the situation.
But Antarctic Research Centre director Tim Naish, based at Victoria University, urged caution.
Prof Naish applauded Prof Flynn for raising the urgency of sea level rise, but said the data he was using was not credible.
Global sea level rise for 2014 and 2015 was about 3.5mm, not the more than 7mm in the NASA data presented, he said.
Even if the NASA data was correct, it was too short a time series to say it would necessarily lead to South Dunedin being underwater by 2034.
Prof Naish said the most credible projections still predicted a 30cm rise by 2065.
Prof Flynn said he made the presentation after seeing news the Dunedin City Council and Otago Regional Council had jointly commissioned an international review by Golder Associates to look at protection options used overseas for rising groundwater.
South Dunedin is protected from the sea by natural sand dunes, but the groundwater linked to the sea level lies just below the surface of many properties and within half a metre for thousands of homes.
'Let's agree it is rising'
The regional council's chief executive, Peter Bodeker, said he had not had time to digest Prof Flynn's new data, but there was no question the groundwater level was rising and action would be required in the suburb.
"Let's not try and estimate whether or not it is the next 20 years or 50 years, let's agree it is rising," said Mr Bodeker.
He said water was already ponding in places in the suburb, and water came up through the stormwater system every spring tide, so groundwater rise was already here, and getting worse.
Mr Bodeker said the Golder report was intended to answer the questions that kept coming up about whether Dutch-style dykes or water drainage systems could work in South Dunedin, or if other solutions had been used successfully overseas.