Site Analysis

Understanding all the features of a site, using and protecting the best, and minimising the impact of the worst.

Rising sea levels

A rise in sea levels as a result of climate change is a real hazard for existing and new buildings in coastal locations. Sea levels around New Zealand have risen by up to 220 mm in the last century and are expected to rise by a further 300–400 mm in just the next 30–40 years.

The IPCC (Intergovernmental Panel on Climate Change) has projected a global average sea-level rise of up to 980 mm before the end of the century. Many houses being built today will still be standing then.

A joint report from the Ministry for the Environment and Stats NZ in October 2017 found that together with rising seas, “We can expect tides, waves, and storm surges (extremely long, slow waves) to reach further inland more regularly, resulting in more frequent and serious flooding…”

Erosion will be a growing problem in many areas, particularly on the east coast of both main islands.

For new house construction this will mean that some locations that could have been built on in the past may no longer be suitable because of:

  • the threat of damage from flooding or erosion
  • inability to get insurance
  • local authority restrictions. (Under the Resource Management Act, local authorities are required to have particular regard to the effects of climate change.)

The inability for some coastal homes to get insurance could affect both new and existing developments. In its Financial Stability Report for November 2018, the Reserve Bank said that “Some insurers in New Zealand appear to have begun adjusting their products and pricing to reflect emerging climate risks, and some existing properties could ultimately become uninsurable."

Insurance industry spokespeople have said that because rising sea levels cannot be considered as accidents or unforeseen events, resulting damage is not insurable.

Where construction is permitted, higher floor levels may be required by local authorities or greater setbacks, such as longer erosion hazard setbacks for cliff-top or beachside homes.

New restrictions may apply in existing settlements. In Mapua and Ruby Bay, for example, there are Residential Closed Zones where further subdivision is prohibited, there can be no land filling, no new habitable buildings and no extension or replacement of existing habitable buildings closer to the shore.

For some existing settlements, choices will have to be made whether to restore natural defences to sea level rise (such as sand dunes or estuaries) or carry out (if possible) a managed retreat of existing buildings, giving up on land that is subject to high levels of flooding or erosion. All of the options have considerable costs. On one of the populated stretches of Hawkes Bay coastline, for example, the cost of work such as sea walls, groynes and managed retreat has been roughly estimated at $130–285 million.

A 2015 study by the Parliamentary Commissioner for the Environment found that there were 43,680 homes and 133,265 people in the higher-risk areas – less than 3 metres above mean high water spring.

The hazard varies by location. Canterbury and Hawke’s Bay have the highest risk exposure, but the challenges are high in other locations too. In South Dunedin, for example, nearly 2,700 homes are less than 500 mm above the spring high-tide mark, with more than 70 percent of homes lying lower than 250 mm elevation. The council is examining how to deal with this, including the option of a “managed retreat”.

A new system for calculating land elevations has made fresh estimates of the numbers of people at risk from rising sea levels. A study published in 2019 indicates that 15,000 New Zealanders live almost on the high tide mark, 71,000 within a metre and 240,000 within 2 metres of the mean high tide line.

Many councils are creating resources and tools to help with planning around flooding and sea level rise. Waikato Regional Council, for example, has created an online Coastal inundation tool that helps give a general indication of the areas that may be at risk. Greater Wellington Regional Council has also produced a website with a dynamic map of areas likely to be affected by sea level rise.

Rising sea levels put infrastructure at risk

A 2019 report by Local Government New Zealand (LGNZ) found that rising sea levels could put $2–14 billion of drinking water, wastewater and stormwater services, roading and other infrastructure at risk.

A rise of 0.5 metres could put roading, the three waters and building infrastructure valued at $2.7 billion at risk. The value rises to many times that figure for higher sea level rises.

These figures do not include the structures that sit on top of the infrastructure such as homes, schools, office buildings and factories.

A summary of the report can be found here and the full report here.


Updated: 11 May 2020