Twelve-metre-high tsunami waves could hit parts of New Zealand's east coast if a 'megathrust' earthquake strikes in the Pacific Ocean, a United States geophysicist says.

Steven Ward from the University of California said the tsunami would have the energy equivalent of 3.6 megatons of TNT, and hit New Zealand's east coast from Christchurch to Auckland.

The initial waves would be felt within 10 minutes of the quake striking, and by the hour waves ranging from five to 12 metres could strike almost the entire east coast of New Zealand.

ingomar200 Up to 12m-high waves crash into the North Island's east coast. Christchurch is hit later, but almost as badly.

The potential risk comes from the Hikurangi subduction zone, a massive fault line running from Marlborough and right past the East Coast where the Pacific and Australian tectonic plates collide.

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It is potentially the largest source of earthquake and tsunami hazards in New Zealand, but scientists say there is still much to learn about it.

JASPER NIJLAND PHOTOGRAPHY Kaikōura would be in the tsunami disaster zone.

Ward, based in Santa Cruz, created a computer program simulation to show how the devastation would envelop New Zealand's east coast by his calculations.

New Zealand sits on the boundary of the Pacific and Australian Plates which have a relative motion of about 4 centimetres a year, he wrote.

In the north, the fault trends offshore and transitions to thrust type which can be a breeding ground for tsunamis.

ALISTAIR HUGHES/STUFF Wairarapa could have as little as seven minutes to prepare for a tsunami from a Hikurangi subduction zone 'megathrust'.

An initial pile of water forms over the offshore uplift, then the pile splits - half heads towards land, and the other half to sea.

Within the hour the massive waves would strike the entire east coast of the North Island, and parts of the South Island.

Ward said the the sites predicted to see the largest tsunami waves would be Christchurch, Wellington and Hawke's Bay.

A multi-million dollar study is already underway on the Hikurangi subduction zone, and scientists believe it has potential to be the country's single biggest geological hazard, yet little is known about the danger it poses.

'Megathrust' quakes in subduction zones have been the cause of major disasters, including the 2004 Boxing Day tsunami that killed 230,000 people around the Indian Ocean, and the 2011 Tohuku tsunami in Japan that left nearly 16,000 people dead.

GNS scientist Ursula Cochran has previously said the effects of a quake on the Hikurangi subduction zone could be devastating.

"We need to think Japan 2011 basically, because if our whole plate boundary ruptured it would be a magnitude-9 earthquake," Cochran said.

"One thing about reflecting on from the Kaikōura earthquake is we don't want people to think this is the big one."

WHAT TO DO IF THERE IS A TSUNAMI:

Civil Defence advises anyone on the coast to leave immediately if there is a long or strong earthquake.

If you are at the coast and experience any of the following:

- Feel a strong earthquake that makes it hard to stand up, or a weak rolling earthquake that lasts a minute or more.

- See a sudden rise or fall in sea level

- Hear loud and unusual noises from the sea

Move immediately to the nearest high ground, or as far inland as you can. Walk or bike if possible. Do not wait for official warnings.

For more information, visit civildefence.govt.nz.