More than 25,000 people live in areas at risk of a tsunami in the Bay of Plenty.
That's according to recently-released research by the National Institute of Water and Atmospheric Research (Niwa). The report was the first national-scale assessment of New Zealand's population and built-environment exposure in tsunami evacuation zones.
Niwa hazards analyst Ryan Paulik, who led the assessment, says nationally 430,000 people live in the zones, of which 170,000 are in red and orange zones.
This included 25,681 in the Bay of Plenty. The region ranked third in terms of population exposure to tsunamis behind Auckland with 34,693 and Canterbury with 30,943.
It excluded people who work, go to school or visit the areas.
The report indicated children and elderly experienced difficulties evacuating and 13,469 people in the over-65 group lived in the Bay of Plenty evacuation zones.
A total of 209sq km of land used for primary production activities such as pastoral farming and horticulture also fell in evacuation zones in the region.
Of the buildings in the region's evacuation zones, 500 were critical buildings - hospitals and government buildings - 3000 were commercial and more than 30,000 were residential.
According to Bay of Plenty Regional Council's emergency management Bay of Plenty director, Clinton Naude, a worst-case scenario could see a tsunami of up to 10m in height reach the Bay of Plenty coastline in less than an hour.
"Anecdotally Bay of Plenty coastal residents seem to have good levels of awareness that if they feel a long or strong earthquake, they should not wait for a warning message, but instead take their essential medications and emergency grab bag, and move to higher ground immediately, preferably on foot."
Naude told the Bay of Plenty Times tsunamis could be generated by underwater landslides or volcanic eruptions but the largest tsunamis were usually caused by underwater earthquakes.
"The greatest risk for the Bay of Plenty region would come from a local source tsunami such as one triggered by an earthquake in the Kermadec Trench in the south Pacific Ocean."
That worst-case scenario would leave little time for risk assessments and official warnings, Naude said, so it was important coastal residents were aware of the natural warning signs and were prepared to act on them.
He said the region's coastal city and district councils had established and signposted tsunami evacuation routes and assembly areas.
"We encourage everyone that lives or works near the coast to do at least one practice of walking their evacuation route so it's familiar to them if they need to use it in an emergency."
However, every tsunami was different, which was why people needed to be aware of all the ways they may be warned, both natural and official, he said.
Each district or city council defines and maps the tsunami zones for its district, based on specific coastal dynamics, geography and other relevant information.
Tauranga City Council resilience and recovery senior adviser Julian Reweti said tsunami modelling looked at land elevation in particular. It also considered the amounts of dune protection, dense housing and trees.
"Water will go to the lowest points more easily, same as when it rains. The Tauranga coast and adjacent land is not truly flat and therefore areas that are slightly lower will allow tsunami waters more easily to flow.
"As you go east towards Te Tumu, the distance that tsunami will travel inland will generally increase."
The worst-case tsunami could put the north of Mount Maunganui to Tay St - apart from Mauao - underwater. In Pāpāmoa, it could come close to reaching the Tauranga Eastern Link.
The primary emergency alerting tools in the Bay of Plenty are the Emergency Mobile Alert system and the Red Cross Hazard App. If a tsunami warning is issued, updates will also be broadcast on local radio stations.
Coastal residents without access to mobile alerting technology should make sure they have arrangements in place with their neighbours or whānau.
Natural warning signs are:
• A loud roar coming from the ocean
• A sudden rise or fall in sea level
Evacuation zones for the Bay of Plenty region:
RED – indicates the marine and beach exclusion zone that people should avoid and/or remove themselves from in the event of any expected tsunami. This represents the highest risk zone and is the first place people should evacuate from in all types of tsunami warnings.
ORANGE – indicates the area that would need to be evacuated in most, if not all, distant and regional-source official tsunami warnings. This would generally be the area that could be affected by a tsunami that takes more than an hour to reach that location from its source, and generally less than 5m in height.
YELLOW – indicates areas at risk from a most credible worst-case scenario locally sourced tsunami. For the Bay of Plenty region that's been estimated as a tsunami up to 10m high, generated from an earthquake in the Kermadec Trench in the south Pacific Ocean.