Tauranga residents whose homes are vulnerable to a major tsunami will be forced to disclose the risk on their property files if regional leaders follow legal advice on the issue.
Mayor Stuart Crosby has revealed a formerly confidential legal opinion that showed the Bay's coastal councils had no option except to record the risk of inundation from a tsunami.
The issue has emerged through a review of the Western Bay's urban growth masterplan SmartGrowth, prompting fears that it could impact on property prices and sales.
SmartGrowth is proposing to put tsunami alerts on land information and property information reports that were used to help people decide whether to buy a house or section.
Tauranga property developers opposed the move, arguing the proposal had implications for thousands of developed low-lying coastal properties in Mount Maunganui and Papamoa, and would flow on to big subdivisions planned for Papamoa East.
The issue was expected to be decided at yesterday's SmartGrowth review meeting but instead debate was postponed pending further work by the councils involved in the growth strategy.
Mr Crosby then revealed his intention to go public with the legal opinion, saying it put a lot more clarity around the obligations of councils. The legal opinion stated if there was a valid risk from a tsunami, then people had to be made aware of the risk through a note on property files. But the risk had to be site specific. Individual properties could not be deemed as being at risk just because they were located in a general tsunami risk area.
Mr Crosby said large numbers of people in Papamoa would not end up getting risk notes on their property files even though they were in a theoretical risk area.
"We should not alarm people and alarm the market."
He said there was still a lot of work to be done, including inundation maps.
The maps would assist in determining the risk, property by property, because there were some high points in the area.
Mr Crosby said there were no statutory grounds on which the risk information could be withheld, but the risk had to be accurate for individual properties. The legal opinion said councils could not face liability because of a tsunami.
Further work on tsunami risks would clarify what should be dealt with by SmartGrowth and the roles of the regional council's Regional Policy Statement and the other councils' district plans, to avoid duplication.
It would also consider the issue of the known science surrounding tsunami risks compared with the advances in forecasting by the time the major population centre of Te Tumu in Papamoa East was developed.
Developers highlighted how earthquake modelling on the Kermadec Trench acknowledged there was uncertainty about the magnitude of earthquakes that would trigger a tsunami.
The trench, which runs from the tip of East Cape to near Samoa, was acknowledged to be the biggest threat to life and property in the Bay because of its 50-minute arrival time.
A Tauranga property developer, who spoke on condition he was not named, said SmartGrowth should be waiting for a national directive because it was a national issue.
He highlighted a media report on April 8 that put Tauranga into context alongside the rest of the country for a one-in-2500-year-return tsunami. It put the city's death toll at 340 from 4.5m wave - not high enough to top the city's sand dunes. "Why should we be jumping the gun?"
New Zealand's leading scientific agency for earthquakes and tsunami played down fears that putting tsunami alerts on property files would scare away potential buyers and reduce values. GNS Science's communications manager John Callan said the fears had not been borne out by overseas evidence.
Papamoa's worst tsunami scenario
Trigger: Earthquake at the southern end of the Kermadec Trench.
Arrival time: 50 minutes.
Size: 13 metres high.
Death toll: Up to 900 people.
Return period for close-in tsunami: 800 years.
GNS Science caution: "Dependent on imperfect data and the variability was large. Would require the entire rupture or huge slips on the trench. It may be that earthquakes required to create this scenario do not occur, but the possibility cannot presently be ruled out."