Sea level rise: 'NZ cannot afford to protect everything we have built'

New Zealand cannot afford to put back everything that will be damaged by rising sea levels, the Climate Change Commissioner says, as new data shows an earlier risk of inundation than expected.

The data shows the sea level is rising twice as fast as previously thought in some parts of Aotearoa, massively reducing the amount of time authorities have to respond.

Globally the sea level is expected to rise about half a metre by 2100 - but for large parts of New Zealand it could more than double that because of land subsidence.

The projections show infrastructure and homes in Auckland, Wellington and many other places risk inundation decades earlier than expected. In the capital, some areas will have a sea level rise of 30cm by 2040.

Climate Change Commissioner Rod Carr said the science had been understood for decades.

"What is new here is the granularity of the science which shows what will happen where and when.

"We've known, now we need to act.

"We do have to make sure that every decision we make about where we build and where we restore infrastructure is seen through a climate change lens.

"We need to understand that New Zealand cannot afford to protect everything we have built and we cannot afford to put back everything that will be damaged."

The government had put out a draft national adaptation plan and Parliament had some years ago put in place the architecture to help New Zealanders understand what needed to be done, he said.

"We need leaders who can lead and we need to support them in making hard decisions about where does the science tell us we need to spend our investment dollars.

In parts of the country defences such as sea walls would be cost effective and appropriate and in others they will "create false hope and wasted resources".

This was a "massive challenge" but the hope was independent scientific evidence can be used to make good choices.

He believed there would be lower-lying communities where people would have to leave their homes within the next 30 years, and the challenge was to use the science and engineering to anticipate what will happen and act in advance.

Managed retreat 'not first port of call ' - Ardern

Prime Minister Jacinda Ardern said the report gave detailed information of the impact of projected sea level rise on coastal parts of New Zealand.

"The reason that's really important for us, is to ensure that alongside local government, planning for the future, we are ensuring that infrastructure or decisions that are being made are taking into account the adaptation that will need be needed in in parts of New Zealand.

"The first port of call isn't necessarily the idea of managed retreat because there are a range of options that can be used."

She said the first steps over the next few months would be working on the national adaptation plan "and making sure that we are working alongside local government and insurers as we work through who bears the cost of some of these."

"We also should not accept that sea level rises going forward into the future are an inevitability, for instance, beyond those that have already been projected in the near term. We should be also making sure that we are doing our bit to do as much as we can to reduce the impacts of climate change."

Retreat a 'traumatic experience'

Local Government New Zealand president Stuart Crosby said retreating from sea level rise should be a last resort.

"We've been adapting for a very long time and the retreat should be very much a last option," he said.

Referring to flood-damaged Matatā he said he and others had been going through a process on 25 to 27 homes in the Bay of Plenty "and I can assure you it's a very traumatic experience for those residents".

Residents or their insurers could "obviously" not on their own pay for managed retreat, and nor could councils, so it had to be a collaborative effort.

In Matatā it was the regional and local council and central government, but it was still an ad hoc approach and had to be a nationwide one.

Eugene Doyle, who lives at Ōwhiro Bay on Wellington's south coast and has been outspoken on the threat posed by the ocean, said communities needed access to better data to be part of the decision-making process.

"As of today we've got a little bit more knowledge about what's coming our way.

"There's a tremendous amount you can do to prepare communities - for example you can have far better warning systems."

In some places, protection work would protect an entire length of coast - but it was important to understand where protection or retreat was needed in communities such as the south coast.

"If you judiciously spend money to harden up the most vulnerable points here, then a much larger community and the whole city gets to enjoy the south coast for a whole lot longer."

Insurance Council chief executive Tim Grafton said there was an urgent need to look how to reduce the risk to homes in priority areas - whether it be changes to buildings or local infrastructure, or retreat.

"If we do nothing ... over time then insurance will price that risk," he said.

"We're not talking about sea level rise, we're talking about the combination of the sea gradually rising and then we have king tides and storms and flood events because the water table close to the beach is at sea level.

"Flood events occur a lot more easily in those circumstances.

Grafton said housebuyers could use maps made available today to make a judgement on the risks. "I'd have a look at that and make some judgements on whether or not I'm prepared to put up with sea level rise hitting my house in 20 or 30 years time ... when you risk manage you have to look at how you control the risk, how you avoid the risk ... and what level of risk we can transfer to insurance."

Community decision

Climate Change Minister James Shaw told Morning Report he wanted legislation to be introduced early next year on the national response to climate change supported by both sides of the House.

"I think this is one of those challenges that this country's facing that is too big for petty partisanship and for small time politics so I will be reaching out to the opposition to say how are we going to work our way through this."

Decisions would have to be made by communities on what course of climate changte adaptation action made most sense for everybody who lived there.

There would probably have to be some sort of dispute resolution, to mediate in communities on options, such as raising houses above flood levels, flood defences or moving people out of communities.