New mapping data has been released showing how rises in sea levels over the next 12, 30 and 110 years could affect coastal residents of Tauranga's inner harbour.

Based on the Government's climate change guidance accepted data, the study was a joint project between the three councils in the Western Bay of Plenty.

The new information has implications for more than a thousand landowners in Tauranga as they decide how much risk they are willing to accept, and what steps they might take to defend their land.

One official acknowledged that the city could lose some houses in the long term.

Letters have been sent to the owners of more than 1500 properties in Tauranga and the Western Bay that may be at risk of erosion from sea-level rise in the next century.

The results of a joint project to map predicted erosion of cliffs and shorelines inside Tauranga Harbour by the Tauranga City, Western Bay of Plenty District and Bay of Plenty Regional councils have been released today.

According to a sobering presentation to Tauranga's elected members, some 640 inner harbour properties in the city may be at risk of erosion by 2030.

By 2130 that number could double under the worst-case scenario modelled, which projected a 1.6m sea level rise over 110 years.

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Some 701 buildings could be at risk, as well as several local roads.

A map of Tauranga showing - in yellow and orange - the erosion risk of the worst-case sea level rise scenario modelled in a new joint council study. Image / Supplied
A map of Tauranga showing - in yellow and orange - the erosion risk of the worst-case sea level rise scenario modelled in a new joint council study. Image / Supplied

Maungatapu, Matapihi and Matua were among the harbourside areas of the city most prone to erosion.

In Maungatapu, the issue of a power pole at risk from coastal erosion has lead to an Environment Court case between Transpower and residents being heard this week.

In the council presentation by Campbell Larking, who led Tauranga's end of the study, elected officials were encouraged to keep their personal views on climate change out of the discussion.

Larking said that of 1243 properties potentially at risk in the long term, 1180 were in private ownership.

The rest were public or council-owned areas such as reserves.

For some, the erosion would have already started.

The range of sea-level rise scenarios for New Zealand to 2130. Graphic / supplied
The range of sea-level rise scenarios for New Zealand to 2130. Graphic / supplied

As legally required, he said the erosion risk would be recorded in each affected property's Land Information Memorandum, and the data would be also be considered in resource and building consent decisions.

The study used aerial maps of the shoreline inside the harbour, as well as visual assessments, to model the erosion risk of nine different scenarios, covering 2030, 2050 and 2130.

The scenarios ranged from extremely unlikely (5 per cent probability) to likely (66 per cent probability) and were based on Government guidance for councils on preparing for climate change and internationally-supported data.

Stats pertaining to the highest recorded temperatures and how our climate is changing.

Coastal strips next to open water, such as Pāpāmoa, were excluded as they have been the subject of earlier studies.

The new information will leave hundreds of landowners facing tough decisions about how much risk they are willing to accept and what steps they might take to mitigate that risk or defend against it.

Local and regional authorities would also be left with decisions to make about their approach to making the city and its residents more resilient to the impacts of rising sea levels, as well as storm surges.

Larking said insurance companies had made it clear they would not pay for climate change.

"Insurance is a business of risk and sea level rise is a sure thing."

Councillor Larry Baldock said the data showed the city was likely to lose some housing.

Councillor Leanne Brown said she would hate to see people panicking or losing sleep over a risk that was years off.

Both the Tauranga and Western Bay councils encouraged people who received a letter to use online tools on their websites.

Tauranga had an interactive map that allowed people to see the projected impact on their property of each of the nine scenarios modelled.

City residents could also book an appointment with a council expert at one of nine community days to talk about their property, while Western Bay residents were encouraged to call the council's customer services team.