A 2013 review by GNS Science predicted waves of 4m-6m were possible along Dunedin’s shores — depending on the source and the size of the quake — up from previous estimates of 3.8m in 2005.
Waves of 6m-8m were possible for outlying coastal communities, and parts of Southland could see 2m-10m waves.
• Calls for fast-tracked tsunami alert system backed
New Zealand’s position on a plate boundary meant it was vulnerable to near-sourced earthquakes and tsunamis, such as that which struck Kaikoura’s coast, which gave little time for a public warning.
The Hikurangi subduction zone beneath Wellington was "arguably" the most significant, and likely to release an earthquake greater than magnitude 8, the report said.
A rare but even larger quake, of greater than magnitude 9, could not be ruled out, which would "produce devastating tsunami similar to those observed in Japan ... in March 2011".
Tsunamis from "regional" sources, like the Puysegur and Kermadec trenches, would give just one to three hours for a public warning before waves arrived.
Those coming from distant sources, such as a large earthquake off Chile, could take up to 14 hours to cross the Pacific and hit New Zealand, giving more time for evacuations.
And, although rare, New Zealand had been struck by "at least 80" tsunamis since 1835, including one in 1855, caused by the rupture of the Wairarapa Fault, which sent a wave into Wellington, the report highlighted.
Parts of Miramar were covered in water 1m deep and waves up to 2.5m high washed into what were then beachside shops in Lambton Quay.
More recently, a magnitude-7.8 earthquake in Fiordland in 2009 sent a tsunami into Dusky Sound, and "significant injury or damage" was avoided only because of the area’s remoteness.
There was also some evidence of a tsunami that struck the Otago coast, and breached sand dunes, sometime in the 14th or 15th centuries, the report said.
But it was not just big quakes that could cause tsunamis.
In 1947, a moderate shake near Gisborne caused no damage but sent an "anomalously large" tsunami crashing into 120km of coast, from Mahia Peninsula northwards, just 30 minutes later.
The "tsunami earthquake", characterised by a slow rupture that caused larger-than-expected waves, was a challenge for public education, as people might not recognise the need to escape quickly, the report said.
New Zealand’s relatively "brief" written historical record — dating back less than 200 years — also gave "very little guidance" on what to expect in the future.
The Indian Ocean (2004), South Pacific (2009) and Japan (2011) tsunamis, which together claimed more than 240,000 lives, were all caused by earthquakes "substantially larger" than previously thought possible at their locations, it said.
That had "overturned many assumptions" about the potential for large earthquakes and tsunamis in the Pacific, with implications for New Zealand, it said.
The greater uncertainty about the maximum magnitude of earthquakes on local faults meant the threats could be "severely" overestimated, or underestimated, it said.
