Rescuers worked frantically to reach survivors under several collapsed buildings in the stricken city centre of Christchurch, after recovering 75 bodies. About 300 people are still missing after Tuesday's 6.3-magnitude quake.

"New Zealanders have woken to a tragedy unfolding in the great city of Christchurch," said Prime Minister John Key, declaring the national emergency.

"The earthquake that struck the Canterbury region at ten to one yesterday has wreaked death and destruction on a dreadful scale."

New Zealand's second city was a scene of "incredible carnage", police said.

Rescuers had to amputate limbs from survivors to free them from smouldering ruins of buildings reduced to debris in minutes, while dazed survivors were plucked from the rubble in a frantic rescue mission that spanned the night.

Four people were pulled alive from the mangled Pyne Gould office building, including one woman who was found hiding under her desk. Rescuers said a group of 15 survivors were trapped inside a collapsed six-storey block.

"We believe we have got a pocket of 15 people," said Ross Ditmer, area commander for New Zealand Fire Service.

Superintendent Russell Gibson said the toll was certain to rise as more than 500 emergency workers combed buildings that had already been weakened by a 7.0-magnitude quake six months ago that miraculously resulted in no deaths.

"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."

Gibson said rescuers were going door to door through the city centre calling out to anyone who was trapped, with efforts concentrating on two city centre office buildings where survivors had managed to communicate with them.

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

Prime Minister Key, who has described the disaster as possibly "New Zealand's darkest day", said dozens of aftershocks continued to rock the city, hampering rescue efforts.

The majority of the city remained without power and Gibson said rescue crews working through the night freed 20-30 people, cutting limbs from some of those pinned in the rubble.

"It's quite amazing, we have some people we've pulled out and they haven't got so much as a scratch on them, we've had other people where we've had to amputate limbs to get them out," he said.

Christchurch resident Anthony Rohan said he survived in his seventh floor office by sheltering under his desk.

"Bookshelves and files were going everywhere, it was just chaos," he told Fairfax Media.

"The force of the earthquake just literally threw you around. Trying to get under desks was a mission...you could see the ground move and the buildings flex."

Others were less fortunate. Tom Brittenden said he saw a woman die with her baby in her arms when she was hit by falling debris in the city's Cashel St Mall. Her baby survived but she was killed instantly.

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

Among those unaccounted for were 11 students and teachers from a language school in the Japanese city of Toyama, Japan's Jiji Press news agency reported.

Other Japanese media reports said several of the students were later rescued alongside a teacher who had emailed her family from underneath the rubble.

Police drafted in search and rescue teams in an attempt to locate survivors, while Japan, Australia and the United States were among countries sending rescuers to help.

Britain's Queen Elizabeth, who is New Zealand's head of state, said she was "utterly shocked" by the disaster while US President Barack Obama offered his "deepest condolences", as expressions of sympathy poured in from around the globe.

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

The tremor hit at lunchtime Tuesday, toppling buildings and leaving central Christchurch strewn with debris. The city's landmark cathedral lost its spire.

Last year's September 4 tremor, measured at 7.0 magnitude, damaged 100,000 homes but killed no one.

Seismologists said that despite being smaller, the latest tremor was more destructive than the earlier quake because it was nearer to Christchurch's centre and much closer to the earth's surface.

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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