Scientists have uncovered evidence that two major earthquakes of the same type that triggered both the 2011 Japanese and 2004 Boxing Day tsunamis hit central New Zealand in the past 1000 years.
And they warn another could cause widespread damage, triggering landslides, sea level changes and a devastating tsunami.
The two subduction "megathrust" earthquakes struck about 10km-30km beneath the seabed in Cook Strait - one of them 520 to 470 years ago, and the other 880 to 800 years ago.
The breakthrough discovery had "confirmed" the risk this type of earthquake, more powerful and potentially destructive than normal quakes, poses to New Zealand.
Both the 2011 Japanese tsunami and 2004 Boxing Day tsunami were caused by subduction earthquakes.
GNS Science earthquake risk analyst Nick Horspool said if a Japanese-style quake of the same magnitude were to hit central New Zealand, thousands of people could be killed and tens of billions of dollars lost.
Mr Horspool estimated the potential impact of such an event using a computer model.
"If it was a magnitude 9, we'd have really strong shaking the whole way up the East Coast of the North Island and even the top of the South Island - intensities near [to] what was experienced in Christchurch.
"There would be widespread damage, damage to buildings and infrastructure, and following that you would then have a tsunami."
He said the tsunami would "devastate" the coastal areas and cause further destruction to buildings and infrastructure.
The exact number of casualties, he said, would depend on how many people were able to self-evacuate.
"Because it's going to be a large, strong shaking... then 10 minutes later there will be the tsunami coming."
Mr Horspool said in the event of a tsunami, wooden buildings were probably more vulnerable than concrete ones.
"We don't have direct evidence from New Zealand events... but what was seen in Japan was that the wood buildings were a lot more vulnerable than the multi-storey concrete buildings."
A combination of elevation above sea level and distance from shore defined the extent of tsunami-prone areas.
Subduction quakes are different from normal earthquakes in that they occur on the under surface of the upper plate - where two tectonic plates meet - instead of on faults within the upper plate.
They have the potential to be bigger in magnitude than fault ruptures closer to the surface, tend to affect larger areas, and are more likely to trigger a tsunami.
The study's lead author, Kate Clark from GNS Science, said the findings were significant as they helped to further understand earthquake and tsunami hazards in the lower North Island and upper South Island.
"Subduction earthquakes are not a 'new' risk for New Zealand, as we have always assumed they can occur, and they are accounted for in our seismic hazard models," said Dr Clark.
"This study is significant in that it confirms that risk."
Although the evidence from just two earthquakes was still not enough to calculate when the next subduction earthquake would occur , or its exact magnitude, she said: "It [was] most likely to be greater than magnitude 7."
It was "fairly likely" a tsunami would be generated, along with widespread coastal subsidence and sea level change, and a significant amount of landslides.
"Certainly Wellington, Napier, Hastings - the larger cities along that east coast of the North Island - and Palmerston North are likely to be affected," she said.
Masterton, Blenheim and up to Gisborne could also be hit.
Scientists have previously found evidence of subduction earthquakes occurring under Hawkes Bay, but this was the first time they had found direct geological evidence that the southern part of the Hikurangi Margin could rupture in an earthquake.
They identified the quakes from sediment cores extracted from Big Lagoon, a large coastal lake east of Blenheim.
The older of the two earthquakes identified at Big Lagoon also triggered a three-metre-high tsunami that travelled inland about 360m.
The research was outlined in the Bulletin of the Seismological Society of America, published this week.
Victoria University Associate Professor John Townend, EQC Fellow in Seismic Studies, said the study provided the first substantiated evidence for large subduction zone earthquakes in the area, and would enable New Zealand's seismic hazard model to be "refined".
"In order to understand the hazard posed to New Zealand by future large earthquakes, we need to know when such earthquakes have occurred in the past - and how big they were.
The challenge with this is that the earthquakes of most interest happen very infrequently."
Wellington region Civil Defence controller Bruce Pepperell said the new research provided "a little more clarity" on earthquake occurrence but would not change safety messages and evacuation plans.
Mr Pepperell said if people felt quakes for a minute or more or were knocked off their feet, that may be the "best and only warnings" of an imminent tsunami.
He said work on the "blue line" project was ongoing. The project produced lines painted across streets showing where the largest tsunamis might reach.
Many low-lying and coastal parts of Wellington were within the blue line or tsunami-risk zone.
Mr Pepperell said it was important communities took part in planning disaster responses and evacuations.
"We really believe if the community comes up with a solution, it's more likely to work, rather than us imposing something on them."
He said "drop, cover and hold" remained the basic safety message for people caught in quakes.
A spokeswoman for Building and Housing Minister Nick Smith said the research would not impact changes announced this month on how earthquake-prone buildings had to be assessed and strengthened.
"This is further research that confirms what experts already knew," she said.
The changes would slash the number of buildings needing assessment from about 500,000 to 30,000, and split the country into three "risk zones" for strengthening.
Affected buildings in the high risk zone, which included Wellington and Christchurch, would need strengthening within 15 years.