New Zealand must better prepare for rising sea levels, according to new research.

At least 9000 homes lie less than 50cm above spring high tide levels, the report from Commissioner for the Environment Jan Wright says.

The Government must allow for the billions of dollars it will cost to move coastal communities to higher ground, her report warns.

“Rising sea levels will have major impacts in many places,” Dr Wright says.

“In time, some coastal land will become uninhabitable.”

Of the major cities, Auckland and Wellington have relatively few homes in low-lying areas, whereas Christchurch has nearly 1000 homes at risk and Dunedin 2600.

Thousands more lie within a metre of the 50cm threshold.

Napier, Whakatane, Tauranga, Motueka and Nelson each have 1000 homes just above the high water mark.

“Continuing sea level rise is not something that might happen – it is already happening, will accelerate and will continue for the indefinite future,” Dr Wright says.

The report says rising seas will bring coastal flooding and could also increase saltwater intrusion into coastal aquifers used as fresh water sources.

Dr Wright recommends a separate policy statement for sea level rise, better measurement of ocean levels and standardised council processes for helping affected communities.

Insurance Council of New Zealand CEO Tim Grafton says the findings are in line with the council’s 15-point plan to protect the country from natural hazards.

“Analysis shows the replacement costs of assets located between 50cm and 150cm from mean high tide levels is $NZ3-$20 billion ($2.7-$18.1 billion),” Mr Grafton said. “Those are huge replacement costs representing thousands of houses and businesses.”

However, the report does not cover the critical issue of underground infrastructure such as stormwater drains, he says. “Many of these are decades old [and] designed to specifications long before anyone was aware of sea level rise combined with climate change, which will bring more intense storm and storm surge.”