A large earthquake could trigger a wall of water more than 12m high, giving people just minutes to get to higher ground.
A tsunami could reach land so quickly there might not be time for sirens to be sounded or a warning to be broadcast.
And the potential for large tsunamis pounding our shores is greater than scientists had expected since the 2011 Japan disaster and the 2004 Boxing Day tsunami rewrote much of the science of tsunamis and the faults that trigger them.
These are some of the conclusions of a report published yesterday on the country's tsunami risk, the Review of Tsunami Hazard in New Zealand, which was carried out by GNS Science, updating a 2005 report.
Civil Defence is now urging people not to wait for a warning but to evacuate immediately after feeling an earthquake so strong they can't stand or that lasts more than two minutes. People should stay in safety until officials give the all-clear.
The report found that because of uncertainty about the maximum size of earthquakes on nearby plate boundaries, tsunamis could be bigger than previously thought.
The 220-page review, which has been two years in the making, considered the biggest tsunami that could be expected in a 2500-year period.
The most hazardous places were found to be Northland, Great Barrier Island, parts of East Cape and Wairarapa where waves of up to 15m above the normal sea level at the shoreline could be expected.
However, there were some spots on the west coast of the North Island where the maximum tsunami height was not expected to exceed 5m - it all depended on geography.
The new report focused on the entire New Zealand coastline, not just the main population centres, using advanced modelling techniques.
Dr Graham Leonard, a natural hazard scientist at GNS Science, said the findings reinforced the need for people to evacuate as quickly as possible because tsunamis had the potential to hit land before a warning could be sounded.
"The single most important message is that if you feel an earthquake longer than a minute or it's hard to stand up, it doesn't need to be both, you need to evacuate straight away and not wait for any official warning. The earthquake itself is the warning."
And people should stay put until an official all-clear is given because often the first wave isn't the largest.
The report found the causes of injuries and deaths from tsunamis were varied but the most common was drowning, people being swept away by fast-moving water and impact from debris.
Survivable injuries often included near-drowning, aspiration pneumonia, and fractures, sprains and strains. A large proportion of tsunami victims were women, the elderly and children who lacked the strength to escape the water or swim against it.
Dr Leonard said the series of major and destructive tsunamis over the past decade had helped raise both public and political awareness of how deadly they could be.
Those three major tsunamis - Japan, Samoa in 2009 and the Boxing Day tsunami - were produced by earthquakes substantially larger than had been considered likely in those locations.
The movement between the tectonic plates in Japan's 2011 Tohoku tsunami was non-uniform and in some areas the plates moved more than 50m, whereas movement was typically 5m to 10m, the report said.
It was also now known there was a similar tsunami in Japan in 869AD, indicating that the interval between the largest earthquakes there was more than 1000 years.
However, Japan's tectonic plates are moving twice as fast as those around New Zealand, suggesting that the interval between the largest earthquakes here could be more than 2000 years.
But because our known earthquake history is so short - just 200 years since the first settlers started recording movements - it can't give scientists much guidance in estimating the magnitude of the largest earthquakes New Zealand may experience.
At a briefing, the report's lead author, Dr William Power, said that meant we were "in a period of greater uncertainty than we thought we were".
As a result of the review, the Ministry for Civil Defence and Emergency Management said it would be emphasising the existing public messages about tsunamis.
It would also encourage councils and the 16 regional Civil Defence groups to reinforce the national public messages with more specific local and regional information and hold regional seminarsduring the next two months.
Auckland Civil Defence controller Clive Manley said the new research meant community response plans would need to be updated.
Expected tsunami heights for northeast and northwest parts of the Auckland region and Great Barrier Island had been raised, as well as those on the coasts of Northland, the Coromandel Peninsula and the Bay of Plenty.
"We will work closely with those communities to help them revise their plans," Mr Manley said.
"Together with our local communities, we will agree what type of signage, markings on the road and information for residents is best for each local area."
Tsunamis in NZ
Wairarapa tsunami, 1855
On January 23, 1855, a magnitude 8.1-8.2 earthquake, the most powerful to strike New Zealand since European settlement, shook the lower North Island and generated a wave with a height of 10m at Te Kopi and 4-5m in several Wellington locations. The first waves struck within minutes. Lambton Quay was hit by a 2.25m wall of water that flooded shops.
Peru-Chile tsunami, 1868
In August 1868, a magnitude 9 quake off the Peru-Chile border caused a tsunami that killed thousands along the South American coast. It spread across the Pacific and became the largest recorded distant tsunami to strike New Zealand. It hit about 15 hours after the quake, affecting ports and causing substantial damage.
Chile tsunami, 1960
A magnitude 9.5 quake struck off Chile on May 22, 1960, resulting in a tsunami that killed thousands in Chile and across the Pacific, including 61 people in Hawaii and 199 in Japan. Thirteen hours after the quake, the first of many tsunami waves arrived on New Zealand's east coast, continuing for three days. In Napier, waves reached 4.5m above high-tide level and damaged a footbridge, wrecked many boats and swept others out to sea.