Emergency services cordoned off central Christchurch, which was devastated by Tuesday's shallow, 6.3-magnitude tremor, to hunt for anyone still alive along with an unknown number of bodies buried in the rubble.

Prime Minister John Key declared a national emergency as 75 bodies were recovered. About 300 people are still missing after New Zealand's worst natural disaster in 80 years.

Key said the quake had "wreaked death and destruction on a dreadful scale" in the country's second biggest city, six months after a 7.0-magnitude quake shook buildings violently in Christchurch but miraculously caused no deaths.

The latest tremor toppled many buildings and left central Christchurch strewn with debris. Christchurch's landmark cathedral lost its spire, and dozens of aftershocks rocked the city, much of which was without power or water.

Rescuers had to amputate limbs to free some survivors, but abandoned hope for any victims in the flattened CTV building, which housed a school for foreign English-language students.

At least 24 Japanese citizens were among the missing, including 11 students at the college along with a South Korean brother and sister in their early 20s.

"This particular site, CTV site, had a number of overseas students in it and my heart goes out to those families that are away knowing that some of their children, family have probably been killed," said police operations commander Dave Lawry.

"The situation is that we don't believe this site is now survivable."

One resident said he saw a woman die with her baby in her arms when she was hit by falling debris, two blocks from the CTV building. She was killed instantly, but the baby survived.

"We tried to pull these big bricks off (her)... she was gone," Tom Brittenden told the Christchurch Press.

Police Superintendent Russell Gibson warned the toll was certain to rise as more than 500 emergency workers combed through shattered buildings, listening for tapping, shouting and other signs of life.

"There is incredible carnage right throughout the city," he told Radio New Zealand. "There are bodies littering the streets, they are trapped in cars and crushed under rubble.

"We are getting texts and tapping sounds from some of these buildings and that's where the focus is at the moment."

Some rescue efforts were frustrated by a two-block exclusion zone around the city's tallest hotel, the Grand Chancellor, as the 26-storey building leaned precariously and looked close to collapse.

But there was applause when a woman wrapped in blankets emerged from the Pyne Gould Corporation building, some 24 hours after the quake rocked busy lunchtime streets on Tuesday.

The survivor's sister, Sally Bodkin-Allen, said her sibling had ducked under a desk when the quake struck.

"It just seems like a miracle... it must be a very strong desk and she must have got under it very quickly," Bodkin-Allen told Fairfax media.

Resident Mark Maynard kept an anxious vigil outside the Pyne Gould building for his wife, who works on the first floor and called 20 minutes before the quake to say she had forgotten her mobile phone.

"It is still no good at the moment. I am hanging around waiting, what do you do?" Maynard told Fairfax.

The clock is ticking for those still trapped, with New Zealand's emergency management chief John Hamilton saying rescuers may have just two or three days to pull out anyone still alive.

The quake was the deadliest to hit New Zealand since 256 people died in a 1931 tremor.

Disaster modelling company AIR Worldwide said the disaster would cost the insurance industry up to NZ$11.5 billion ($8.6 billion).

Specialist teams from Australia, Britain, Japan, Singapore, South Korea, Taiwan and the United States were due to join the rescue operation.

Irish authorities said one Irish citizen and one British citizen were among the dead and there were "serious concerns" about two others.

Condolence messages came from the Vatican and from exiled Tibetan spiritual leader the Dalai Lama.

The Pope called on the faithful to "pray for all those who lost their lives" and asked God to help rescuers.

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.

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