A tsunami triggered by a massive earthquake along the southern section of the Kermadec Trench could kill up to 900 Papamoa people and cause $54 million of damage to buildings, a new report says.
The losses relate to a worst-case scenario for a 13m-high tsunami with a 50-minute arrival time. Kermadec Trench was a "significant" tsunami risk to the Bay of Plenty, running northeast of East Cape towards Tonga.
The Bay of Plenty Regional Council commissioned the report to help guide decisions on developing the satellite city planned on farmland along Papamoa's low-lying coastal plains to Kaituna River.
Its predictions included all of Papamoa's built-up areas because the first phase of the satellite city, Wairakei, lay behind the existing houses east of Parton Rd. The second phase, Te Tumu, took development to the Kaituna.
The report looked at scenarios including the tsunami's size, how far away it was generated, and the intensity of future development in Wairakei and Te Tumu.
With Papamoa already largely developed in low-density housing, the report predicted that 905 people - 6 per cent of the population - would die if the tsunami arrived at night. A day-time tsunami would take 446 lives.
Papamoa was at greatest risk because Wairakei was further inland and Te Puna had a better dune system to protect it.
A total of 1480 homes would be damaged to a value of $54 million in Papamoa.
The report's good news was that a tsunami generated locally, such as from White Island or the Astrolabe fault, would reach a maximum height of 3m and not top the dunes. A tsunami generated by a quake with a magnitude less than 8.5 along sections of the Kermadec Trench was unlikely to result in a large number of fatalities in Papamoa, Wairakei or Te Tumu. A tsunami from distant sources such as South America would have a maximum wave height of 2.5m to 5m - also not high enough to pass over the dunes. But it would impact on low-lying areas behind the Maketu Estuary and the mouth of the Kaituna River.
Distant tsunami rolled into the Bay on average every 50 years whereas a large one had a probability of one per 1000 years. The report said 20 to 50 deaths and injuries were expected from a 100-year tsunami.
Te Tumu was the only area at risk of deaths from a smaller tsunami because a surge up the Kaituna River would inundate parts of the planned suburb.
The report said the tsunami which caused an intolerable level of deaths would require either the entire rupture or huge slips of the Kermadec Trench.
"It may be the case that earthquakes required to create these scenarios do not occur, but the possibility cannot presently be ruled out." The study authors said they were dependent on imperfect data and the variability was large.
Papamoa resident Wayne Rose told the Bay of Plenty Times the projected death toll was alarming but believed such an event was unlikely. "It doesn't worry me a hell of a lot. It's a bit of a worry if it could kill that many people but personally I don't think there's much chance of that."
Papamoa Progressive Association chairman Steve Morris said the $54 million of building damage sounded conservative. But he supported the thrust to put tsunami into the risk-based approach to planning for natural hazards.