Nearly 25,000 Christchurch properties could be at risk of future seawater inundation, new estimates show.
A new Tonkin and Taylor report shows that under a worst case climate change scenario, flooding could affect nearly 25,000 coastal city and Banks Peninsula properties in the next 50 to 100 years.
Its 2015 report identified 18,000 properties at risk of coastal inundation plus 6000 potentially susceptible to erosion.
That report was criticised as scientifically flawed, which resulted in the updated 2017 report. It shows a similar amount of properties in total are vulnerable to coastal hazards, but far fewer to erosion.
Lobby group Christchurch Coastal Residents United (CCRU) said the city council must do more to help ratepayers whose properties were at risk, while city councillor Glenn Livingstone urged seaside residents not to "panic".
"There is a future for the city, there is a future for the east of the city," he said.
Councillors last week received the 2017 Coastal Hazards Assessment for Christchurch and Banks Peninsula.
City council staff told councillors the "maximum extent" of inundation, out to the year 2120, was "somewhere around about 25,000 properties".
The 2017 report showed around 1000 properties were vulnerable to erosion, far fewer than the 6000 in the 2015 report. Many of those were at risk of inundation, however, which is reflected in the new report.
The 50-year sea level rise projections ranged from 30 centimetres to 55cm depending on different climate change scenarios. The 100-year projections go from 55cm to 1.36 metres.
The council is legally required to note coastal hazard risks on Land Information Memorandum (LIM) reports. When it did so following the 2015 report, it was met with a backlash by some coastal residents, who asked they be removed until a new report was completed.
The references to coastal hazard risks were changed, but not removed.
Christchurch is the New Zealand city most exposed to climate change – it has $3b of buildings in areas prone to flooding, according to a NIWA risk exposure report.
Along with those in coastal suburbs such as Sumner and Southshore, people living along the lower Avon and Heathcote rivers are increasingly exposed to flood risk.
CCRU spokesman Darrell Latham said the focus needed to be on adaptation.
"Affected communities shoulder the risk and effects of climate change. However, climate change is a national responsibility not just the responsibility of coastal communities or local councils.
"The new coalition Government needs to step up and work with councils on the full range of scenario implications for affected communities.
"Mitigation and compensation must be part of the conversation," Latham said.
Responding to climate change issues has largely been left to councils, but local authorities have pressured central Government to take a larger role.
It was important the city council backed communities, instead of continuing with the "easy technique of walking away by issuing section 72 notifications", Latham said.
Section 72 of the Building Act says a council must grant a consent if it considers the work will not accelerate, worsen, or cause a natural hazard.
He said: "Residents are directly affected and carry all the responsibility and potential financial and insurance implications. "This approach is not fair nor is it sustainable."
CCRU had concerns about aspects of the 2017 report, Latham said. The 2015 report, under a worst case scenario, projected 1.08m sea-level rise by 2100 – the 2017 report moves that projection to 2120.
"The question needs to be asked, 'why so?' Why are they moving the goal posts and is it because current predictions to date have not materialised?" he said.
CCRU requested information on current sea level rise in the new report and was "extremely disappointed" that was not included. "This information could be used as a pragmatic basis for comparison," Latham said.
Livingstone said there was no need for Christchurch residents to panic as there was still "time".
"But we do need to take seriously our future as a nation as well as locally," he said.
Deputy Mayor Andrew Turner said councillors were confident the 2017 report met the recommendations the independent peer review panel made for changes and further investigations from the 2015 report.
"Our focus now is to have meaningful conversations with our communities and explore the options to adapt to the challenges of a changing climate and sea-level rise," Turner said.