New Zealand's Christchurch weathered a 7.0 earthquake, but a smaller 6.3 aftershock toppled buildings and killed scores largely because it was a "bullseye" direct hit, scientists said.

Tuesday's cataclysmic tremor, which left nearly 400 people dead or missing and the city centre in ruins, was so close to the city of 390,000 and so shallow that major damage was inevitable, they said.

"This quake was pretty much a bullseye," said Professor John Wilson, deputy dean of engineering at Australia's Swinburne University of Technology.

"It was quite a large 6.3-magnitude event and so close to Christchurch that we weren't surprised to see significant damage. At that close range, the level of shaking is quite severe."

The earthquake struck six months after the violent 7.0 tremor damaged 100,000 buildings and left a major repair bill, but caused no deaths, after striking overnight on September 4, when most people were safely in bed.

But this week's tremor hit at the worst possible time, at lunch on a weekday, when offices were open and streets were busy with shoppers who were vulnerable to falling masonry.

Its epicentre was only five kilometres (three miles) from the city at a depth of just four kilometres below the land's surface, meaning there was little ground to absorb the blow.

Some of the worst-hit buildings, including Christchurch's landmark cathedral -- which lost its spire -- and The Press Building housing the local newspaper, were historic structures in the city's heart.

However, newer office blocks such as the CTV and Pyne Gould buildings collapsed, while the towering Grand Chancellor Hotel was tottering dangerously. New Zealand buildings have been designed to resist earthquakes since the 1970s.

"We expected the older buildings with unreinforced masonry to suffer -- their masonry is heavy, brittle and vulnerable to earthquake shaking," Wilson said.

"In general the contemporary buildings performed well, although a few contemporary buildings have collapsed, which did surprise us."

David Rothery, of the Volcano Dynamics Group at Britain's Open University, said the soft ground on which the city is built would have magnified the shaking, making the 6.3 quake even more deadly.

"In much of Christchurch where the ground is flat and underlain by sand or silt, some structures have been shaken apart, causing upper stories to collapse onto the floors below," he said.

"This is because soft ground magnifies how violently the surface shakes during an earthquake."

Australian Seismological Centre director Kevin McCue also said the tremor could increase pressure on plate boundaries across New Zealand, increasing the likelihood of a tremor elsewhere, particularly in the capital Wellington.

"If you have one (quake) it ups the hazard," he told the New Zealand Herald.

"This quake has the potential to load up the plate boundary, increasing the likelihood of a quake at Wellington."

"Wellington has always been considered much more at risk because it straddles the plate boundary. New Zealand has been relatively quiet since the 1930s -- maybe (it's) about to catch up."

New Zealand sits on the "Pacific Ring of Fire", a vast zone of seismic and volcanic activity stretching from Chile on one side to Japan and Indonesia on the other.

Tuesday's quake is the most deadly to hit New Zealand since a 7.8-magnitude tremor killed 256 people in the Hawke's Bay region in 1931.