Local councils need to start planning now for dramatic sea level rise, a new report warns.
The stark warning comes in Ministry for the Environment advice to local government on preparing for "coastal change", and it's coupled with an admission by Climate Minister James Shaw that New Zealand "currently lacks a coordinated plan on how to adapt to climate change."
According to Environment officials' projections, in a worst case scenario, sea levels could rise by almost a metre by 2060. Within a century (2120), that could be by as much as 1.36m.
Just over 133,000 people in coastal areas could be affected.
More than 68,000 buildings are at risk - and the total cost of replacing them is $19b dollars. That includes 382 "critical facility" buildings, five airports, more than 1500 jetties and wharves, 46km of railway and more than 2000km of roads.
The hardest hit areas are in Canterbury and Hawkes Bay - with Waikato having the greatest length of roads exposed.
Wastewater treatment plants, potable water supplies and stormwater and overland drainage systems will all be affected.
"Ongoing sea-level rise will lead to irreversible impacts at the coast ... because many land-use planning and asset and infrastructure decisions made today have long lifetimes because of the permanency of development (eg, subdivision, buildings and infrastructure), planning for adaptation at the coast needs to start now," the report reads.
It continues: "Over time, however, communities will be left increasingly exposed, with vulnerable assets and a stock of private and public investment (eg, buildings, roads, utility services, sea walls) for which difficult decisions will be required - remove, relocate or demolish, or invest substantially to protect.
"The places and environments valued by people will also be exposed to increasing impacts, and vulnerable groups and those without the capacity to move will be particularly affected.
"The likely scale, extent and impact of the evolving increase in coastal risk will be unprecedented across New Zealand."
The report also says sea levels are expected to keep rising for several centuries, even if global gas emissions are reduced, and we can expect more dramatic weather events causing high tides.
If the sea level rises by between 0.3 and 0.4 metres - possibly by 2050 - we can expect rare storm tide inundations to happen at least once per year.
"In New Zealand, by 2050-2070, extreme coastal water levels that are currently expected to be reached or exceeded only once every 100 years (on average) will occur at least once per year or more."
Climate Change minister James Shaw says the report shows "the size of the task to build New Zealand's resilience to rising sea levels, a warmer climate, extreme weather and other impacts of climate change".
"It's important that New Zealanders have a clear picture of the potential impacts of climate change so that communities, local and central government, business and other sectors of our economy can make well-informed decisions about how we build resilience and adapt," he said.
Mr Shaw says the country is "in the early stages of planning" but that it currently lacks a coordinated plan on how to adapt to climate change.
The Government's Climate Change Adaptation Technical Working Group will report back with recommendations on adapting to the impacts of climate change in March.