Experts say that the impact of sea-level rise will become clear long before the sea washes over the land. And it will probably start underground.

New Zealand has a vast network of underground stormwater and wastewater pipes, many of which were designed long ago and have aged considerably since they were installed. Like the communities around them, they were built near the coast, under the assumption that the sea would stay where it was.

Most of the pipe network is gravity fed, meaning the pipes start high and tilt downwards, ending at the lowest point, usually at the coast or near a river to discharge into water. They are particularly exposed to sea-level rise, simply because they are closer to the sea, or tidally influenced rivers, than anything else.

When the sea rises, corrosive saltwater is more likely to reach the ends of the pipes and infiltrate the network, causing flooding and pipe damage. In some areas, such as South Dunedin, the integrity of the pipe system is crucial.

South Dunedin is more exposed to sea-level rise than any other large community, despite the fact it's not connected to the sea. The problem there is groundwater; like the sea, it rises and falls with the tides, and has been steadily rising.

Cleaning up groundwater flooding in South Dunedin. Cleaning up groundwater flooding in South Dunedin.

Because the groundwater is so high, rain has nowhere to go, and ponds on the surface. If the pipes break, the water will amass.

Adding to this problem is that rain events are projected to become more extreme in most parts of New Zealand, particularly in winter months. When high seas, storm surges, and heavy rain combine, the effect can be too much for existing infrastructure to handle.

Replacing these at-risk assets will be a heavy burden for local authorities - the value of the pipes at risk is unknown, but those most likely to be affected include large areas of Christchurch and Wellington.

If they are not fixed, or moved, broken pipes can cause a variety of environmental and health issues. Another issue is that it would mean greater use of pumps, which are expensive, and usually run on diesel, which further pollutes the atmosphere.

Above land, a major issue won't be infrastructure, but finance.

Insurance, in the most basic sense, is protection against uncertainty; it's a way to transfer low-probability risks, like damage from a fire, an earthquake, or a major flood to someone else.

By definition, insurance covers events unlikely to happen. That the sea will rise is certain.

There is clear evidence that extreme weather events causing damage are becoming both more severe and more frequent. The Insurance Council releases its claims data every year, which show that storms causing tens of millions of dollars in damage now happen several times a year.

The greatest threat is when a community as a whole loses insurance, a phenomenon known as "insurance retreat".

There is no evidence this has happened anywhere in New Zealand, but there are growing signs it will. Insurance premiums have risen in some areas following major storms, like the one that hit Edgecumbe in 2016. When premiums start to rise more broadly in coastal areas, it's a price signal - the market is saying that living on the coast is riskier than it used to be.

Sea-level rise researchers believe insurance retreat will be among the first major consequences of sea-level rise in New Zealand, because it unleashes a cascade of other issues.

For insurers, it's a mathematical problem: At what point does an unlikely event become too likely, turning a profit into a loss? Insurers do their own risk modelling, and likely have a sense of when and where retreat will happen, but are reluctant to share that data.

It has left policy makers and researchers working on their own estimates.

Belinda Storey, a researcher from the Victoria University of Wellington, is examining the point at which insurance retreat is likely around the country. She says it's likely sooner than many might expect.

"Insurance won't provide cover for a certainty; they'll only provide for probability," she says.

"Once that probability reaches a point where it's very likely, then they simply won't provide insurance at all. So insurers are going to pull out of those locations that are most affected by climate change well before people are ready to move."

A major problem is that insurance policies are annual, but the mortgages on the houses they apply to last decades. A house can have insurance one year and not the next, but its mortgage will remain.

That shackles insurance to the banking sector: When a house loses insurance, it leads to a technical default, because having insurance is often a requirement for a mortgage. The bank is left with the house, which has likely lost value.

For the person living in the house, getting another mortgage is tricky, because they can't use an uninsured property as collateral for a loan. If the house is damaged in a storm, they'll have to pay the repairs; without private insurance, they are not covered by EQC, either. Continuing to protect the house may require costly defences, such as a sea-wall, paid for out of pocket or through targeted rates. The cost of coastal living grows, while the value of their asset drops.

The local authority, which makes infrastructure decisions based on long time-periods, may decide to stop investing in the area; the roads decay and the pipes rot. There's a possibility that the only people living on the coast become those with no other options.

"You may have people moving into those locations who can't afford to live anywhere else," Storey says.