Councils could be liable for up to $13 billion to replace infrastructure - or "far beyond" that figure - if measures aren't taken to prevent or adapt to sea level rises right away, according to a new report.
And the cost to the private sector could run into tens of billions, insurers have warned.
The report - Vulnerable: The Quantum of Local Government Infrastructure Exposed to Sea Level Rise - was commissioned by Local Government NZ (LGNZ) and released on Friday morning.
"As this study highlights, roughly 65 per cent of New Zealanders live within five kilometres of the ocean, and an increasing number of councils will face significant policy and financial challenges as the sea continues to encroach on them," said LGNZ president Dave Cull.
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If the sea level rises 1.5m by 2100 - which is at the higher end of scientists' predictions - the report says the cost of replacing council infrastructure would be $8 billion. At 3m - which isn't outside the realms of probability - this would rise to $13 billion.
Low-lying Canterbury will be hardest-hit, followed by Hawke's Bay, Auckland, Wellington, Bay of Plenty, Otago and Waikato.
Almost half the total would need to be spent replacing swamped drinking, storm and waste water assets. If the sea gets higher, bridges, landfills, green space, airports and treatment plants would also be affected.
Even the almost-certain rise of only 0.5m would see $1.4 billion in damage done to council infrastructure.
A 1.5m rise would see 2100km of local roads underwater, and 3m would drown 4559km, with a replacement cost of $2.3 billion - and this doesn't include bridges.
"This report should have everyone sitting up and listening," said chief executive of the Insurance Council of NZ Tim Grafton, adding that others stand to lose even more.
"We believe the full cost of exposure to central government and private sector property will be in the tens of billions of dollars... This is not just an issue for local government. Central government has a key role to play too and ultimately carries the economic, social and political risk if adapting to sea-level rise is not well managed."
Minister for Climate Change James Shaw said the report highlights the "sheer magnitude of devastation" sea level rises can cause.
"It would destroy critical infrastructure that allows our communities to function and thrive. The consequences of that are frightening. To delay or weaken our stance would be to the detriment of New Zealand communities."
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The report says we have "roughly 25 years" to get ready before all that will be left to do is park an "ambulance at the bottom of a metaphorical hill".
"Costs will likely go far beyond tangible measures; not only will infrastructure be exposed, so will potential economic development and growth, community health and safety, and social support systems."
Actions proposed by LGNZ to mitigate and adapt to the threat include:
- LGNZ leading a "national conversation" about what can be expected
- establishing a National Climate Change Adaptation Fund "to improve stakeholder participation in responding to climate change to ensure equitable outcomes"
- forming a Local Government Risk Agency to "assist and guide consistent and expedited planning, decision-making and procurement, and build local government capability and capacity to identify, quantify and understand risk"
- and create a National Master Plan so options can be prioritised across the country.
Mr Shaw says the Government isn't wasting any time.
"The challenges can be met, and the best way to do that is to gather the best information we can, along with the best community involvement and expert advice possible to put strategies in place sooner rather than later."
The Insurance Council says every dollar invested now "to reduce and adapt to risk" will save many more in the future.
Earlier this week, the West Coast Regional Council made a submission against the Government's Zero Carbon Bill, saying it needed proof climate change was being caused by humans.
The overwhelming majority of climate scientists say it is.