Civil Defence says there is a potential threat of a tsunami reaching New Zealand after a magnitude 8.9 earthquake struck Japan tonight, NZ time, but it has not issued an official warning yet.

"A tsunami is possible. MCDEM (Ministry of Civil Defence & Emergency Management) and scientific advisors are assessing the severity of the threat to New Zealand," Civil Defence said.

The Pacific Tsunami Warning Centre (PTWC) has issued a tsunami warning for New Zealand in response to the quake.

The New Zealand Tsunami Expert Panel assessment is that this is a marine threat only for New Zealand - meaning strong and unusual currents are possible in the sea, rivers and estuaries. No land threat is expected at this time.

The Centre gave an estimated arrival time for any tsunami of 6.14am at the North Cape (NZ time Saturday).

People in New Zealand coastal areas were warned to stay off beaches and out of the water.

A 4-metre tsunami washed away cars and tore away buildings along Japan's eastern coast near the epicentre after the earthquake hit at 2.46pm (6.46pm NZT).

There are an estimated 100 New Zealanders in the northeastern part of Japan worst-affected by the quake and tsunami, a Ministry of Foreign Affairs and Trade spokeswoman said.

The number was an estimate because not all travellers registered with the embassy.

There were 3500 New Zealand registered as travelling or living in the whole of Japan and about 1600 of those were known to be in Tokyo.

The New Zealand embassy in Tokyo had reported no casualties among its staff, but the spokeswoman said there had not yet been any contact with Bunzo Kamei, a businessman who is consul in Sendai, one of the worst-affected cities. That city has a population of 1 million, and is about 360km north of Tokyo.

The quake that struck was followed by a series of powerful aftershocks, including a 7.4-magnitude one about 30 minutes later. The US Geological Survey upgraded the strength of the first quake to a magnitude 8.9, while Japan's meteorological agency measured it at 7.9.

The meteorological agency issued a tsunami warning for the entire Pacific coast of Japan. NHK was warning those near the coast to get to safer ground.

The quake struck at a depth of 10km, about 125km off the eastern coast, the agency said. The area is 380km northeast of Tokyo.

In downtown Tokyo, large buildings shook violently and workers poured into the street for safety, and TV footage showed fires across the city in buildings and at an oil refinery.

A New Zealander who is co-director of the Australian Tsunami Research Centre and Natural Hazards Research Lab at NSW University, Professor James Goff, said that though Japan had a rigorous earthquake building code and excellent tsunami warning system and evacuation plans the powerful and relatively shallow quake would likely provide a severe test.

An Australian seismologist based in Canberra, Kevin McCue, said the quake was the largest recorded in Japan.

There had been seven earthquakes in Japan over magnitude 8 since 1891.

In 1923 the 7.9 magnitude Kanto earthquake killed 147,000 people, and Mr McCue said it was likely that many people had been killed in the latest quake, even though it was further north of Tokyo than the Kanto earthquake.

- NZPA, NZ Herald staff