Thousands of seaside homes around New Zealand could face soaring insurance premiums - or even have some cover pulled altogether - within 15 years.

That's the stark warning from a major new report assessing how insurers might be forced to confront the nation's increasing exposure to rising seas - sparking pleas for urgent Government action.

Nationally, about 450,000 homes that currently sit within a kilometre of the coast are likely to be hit be a combination of sea level rise and more frequent and intense storms under climate change.

The findings coincide with Prime Minister Jacinda Ardern poised to declare a climate change emergency in Parliament today.

The report, published through the Government-funded Deep South Challenge, looked at the risk for around 10,000 homes in Auckland, Wellington, Christchurch and Dunedin that lie in one-in-100-year coastal flood zones.

That risk is expected to increase quickly.

In Wellington, only another 10cm of sea level rise - expected by 2040 - could push up the probability of a flood five-fold - making it a one-in-20-year event.

International experience and indications from New Zealand's insurance industry suggest companies start pulling out of insuring properties when disasters like floods become one-in-50-year events.

By the time that exposure has risen to one-in-20-year occurrences, the cost of insurance premiums and excesses will have climbed sharply - if insurance could be renewed at all.

The report found about 540 exposed homes in Auckland, with current median coastal flood premiums for once-in-a-century events of $2000, could reach that one-in-20-year threshold with just 15cm of sea level rise.

If insurance was still available at that point, premiums would have soared to $10,000.

For the 1740 Wellington homes modelled, median premiums could jump from $1800 to $8700 with seas 12cm higher.

In the South Island, 4850 properties in Christchurch and 3100 in Dunedin, could see premiums leap from $1600 to $7600 and $7900, with 13cm and 14cm of sea level rise respectively.

The research also suggested that just small increases in sea level would likely cause at least partial retreat by insurers for most of those 10,000 homes, within only 15 years.

Insurance remained a requirement for residential mortgages, and failing to maintain them could trigger defaults.

While mortgages were often granted with repayment periods of up to 30 years in New Zealand, insurance contracts were renewed annually.

An insurer could exit a market within 12 months, while a lender could still have decades before their loans matured.

Despite rules requiring mortgagors to insure, a general absence of compliance checks meant banks currently didn't know whether some properties they mortgaged remain insured beyond the first year of ownership.

Insurance Council of New Zealand chief executive Tim Grafton expected climate risks would continue to be signalled by insurers through price and excesses.

"Insurers are likely to continue to cover existing homeowners in climate change or natural hazard-threatened areas, but we can expect premiums and excesses will increase," he said.

"That might mean a floodplain property will have a $500 excess for fire damage but a $10,000 excess for flood damage.

"However, some companies might choose not to take on new customers in areas that have a known higher risk to sea level rise and flooding in 15 to 20 years."

Insurers needed to work with customers to ensure they understood the reasons and costs that came with pricing certain risks, he said.

About 450,000 homes that currently sit within a kilometre of the coast are likely to be hit be a combination of sea level rise and more frequent and intense storms. Photo / Paul Taylor

Meanwhile, homeowners were still choosing to buy, develop and renovate coastal property, and new houses were being built in climate-risky locations, said the report's lead author, Dr Belinda Storey of Climate Sigma.

"People tend to be very good at ignoring low-probability events.

"This has been noticed internationally, even when there is significant risk facing a property.

"Although these events, such as flooding, are devastating, the low probability makes people think they're a long way off."

Storey felt that market signals weren't enough to effect change - and the Government could play a bigger role informing homeowners of risk.

Grafton said the sector continued to push for adaptation measures and backed a recent recommendation for the Government to create a dedicated Climate Change and Managed Retreat Act.

"Some companies might choose not to take on new customers in areas that have a known higher risk to sea level rise and flooding in 15 to 20 years," ICNZ chief Tim Grafton says. Photo / Supplied

"We also strongly advocate for local government to take a long view out 50 to 100 years and not to consent to developments in high risk areas," he said.

"The sooner we adapt to our changing climate, the less adaptation will cost us and the less we will be impacted by the increasing frequency and severity of storms."

Local Government New Zealand agreed clearer rules, guiding where people could live and build, would also help councils currently "stuck between a rock and sea level rise".

"Certainty in the adaptation space would also help insurers, and make their business model more sustainable and less prone to sudden swings in risk pricing," a spokesperson said.

Climate Change Minister James Shaw said nationwide risk had been mapped out by an assessment published this year.

"Based on this, we are currently working on a National Adaptation Plan, which will set out how we plan to ensure communities are more resilient to the impacts of climate change."

By Jamie Morton, NZ Herald