New Zealand Prime Minister John Key has declared a national state of emergency  in the wake of a powerful  6.3 magnitude earthquake.

Superintendent David Cliff says police are systematically making their way through the rubble looking for survivors.

"There are still people trapped in the rubble who are texting loved ones and keeping us informed," Supt Cliff told Channel 9.

He said the pancaked CTV building was of particular concern, with many people still trapped in there.

"We want to systematically go through the city and look in the rubble," he said. "It has been really heart wrenching - we know there are bodies, we know there are deceased but our priority has to be with the living."

Superintendent Russell Gibson said bodies still littered the streets of New Zealand's second city after Tuesday's 6.3-magnitude quake and the earlier estimated death toll of 65 was set to rise significantly.

"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 the number of trapped "could be another 100, it could easily be more than that".

"It will be significantly higher than that," he said.

More than 500 rescuers, including police and military personnel, pulled between 20 and 30 people from the debris overnight, toiling through the darkness, he added.

"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.

Gibson said emergency workers were going door to door through the city centre calling out to anyone who was trapped, with rescue efforts concentrating on two 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.

More than 200 patients have been admitted to Christchurch Hospital's emergency department, but doctors have not been able to save everyone.

Most people were admitted with lacerations, head injuries and broken bones after being hit by falling bits and crushed under masonry, the hospital's acting emergency department head Mike Ardagh said.

But as the night went on people were admitted who had been trapped and had more serious injuries.

"We've got a number of those sorts of patients where the injuries might be harbouring something more dangerous," he told Radio New Zealand.

"Some have sadly died, most of the deaths you will hear in the numbers didn't come to us at all."

Hospital staff were left working in the dark for some of the night because of power cuts.

"(It was) pretty unusual circumstances. Everyone rolls their sleeves up and gets on with it."

New Zealand Prime Minister John Key announced last night that “at least 65 people have lost their lives” and noted rescuers were still searching through the ruins of collapsed buildings looking for injured and trapped survivors - and bodies.

Christchurch Mayor Bob Parker said more than 100 people were trapped in about six sites - but it was possible more were trapped in individual houses.

Parker told Radio New Zealand up to 200 people could be trapped, while local TV3 news reports said 125 people had been rescued.

Police drafted in urban search and rescue teams in an urgent attempt to locate survivors.

“It is a just a scene of utter devastation,” Key said after rushing to the city within hours of the quake.

“We may be witnessing New Zealand's darkest day,” he said.

“People are just sitting on the side of the road, their heads in their hands. This is a community that is absolutely in agony,” Key said, warning that the toll was likely to rise.

New Zealand's deadliest tremor in 80 years struck as city streets were packed with lunchtime shoppers, and turned central Christchurch into a rubble-strewn disaster zone littered with dazed and bleeding residents.

The city's iconic cathedral lost its spire, and the multi-storey Canterbury TV building was reduced to a smoking ruin.

Rescue helicopters plucked survivors to safety from the rooftops of buildings where staircases had collapsed, and emergency workers used giant cranes to pull office workers out of ruined city buildings.

It was the second major quake to hit Christchurch in five months, though yesterday's shallow quake caused far more destruction than the more powerful September quake that struck before dawn on a weekend.

Police have reported “multiple fatalities” at several locations in the downtown area, including where two buses were crushed by falling buildings.

The Pyne Gould Corp building, several storeys high, folded up like a pack of cards and rescuers were still trying to find trapped occupants last night. Up to 50 people were said to be in the wreckage - alive or dead.

Rescuers were trying to get people out of the Canterbury TV building, while firefighters battled a fire there.

Graphic photographs on television showed blood-splattered survivors scrambling from downed buildings or crawling from under shattered shop verandahs that had fallen on to city footpaths.

Crushed cars lined parking spaces, masonry scattered across roads.

Victims were crushed to death as buildings collapsed, many of them weakened in last year's event.

TV3 said dead bodies had been pulled from a hostel and a bookshop, and that a tourist was crushed to death in a van.

All flights in the country were briefly suspended after a Christchurch control tower was damaged. About half of Christchurch remained without power last night. Earlier in the day, mobile phone networks were disrupted and road and rail transport was badly hit after the violent tremor, which tore gaping fissures in asphalt.

Water mains were also burst, unleashing a torrent of water that joined with heavy rains to inundate the suburb of Bexley and cause surface flooding in other areas of the city.

Video footage showed a landslide crushing a small building, while passersby fled for their lives from underneath a collapsing awning.

“This is about as bad as it gets,” said Parker, the mayor, who declared a five-day state of emergency and said emergency crews would work through the night to reach the dozens of trapped people.

“What the picture will be in the morning, God only knows,” he added.

The death toll is already the second highest in a New Zealand earthquake - outranked only by the 256 people who died in the violent 7.9 1931 Hawke's Bay quake. Scientists put the epicentre at 10km southeast of the city - at Lyttelton, the city's coastal port - at a depth of only 5km.

Radio and television reported damage in the town of Lyttelton was severe.

Key flew to Christchurch yesterday afternoon and, after a quick tour of the city, described it as “utterly wrecked”, adding “this is an absolute tragedy for Christchurch”.

Parker, lauded for his behaviour and slick civic control in the wake of last year's quake, said the damage today was much more severe.

“The city centre is like a war zone and damage is immense. Everybody needs to understand that this is going to be a day of very black news,” he said.

Christchurch Airport closed after the earthquake and was to remain shut down overnight, open only to emergency flights and aircraft carrying rescuers and medical helpers.

Extra police, armed forces troops and search and rescue teams were heading for Christchurch last night, while a NSW search and rescue team was due to start work at first light this morning.

Heavy aftershocks continued to roll across Christchurch last night, further frightening shocked residents. A 5.0 magnitude jolt hit at 7.43pm local time (5.43pm AEDT).

Fears for Australians in quake

AUSTRALIAN Consular officials hold fears for Australians who were in Christchurch at the time of yesterday's earthquake.

The Department of Foreign Affairs and Trade (DFAT) says no details are available yet about the nationalities of victims.

But Australians need to prepare for the possibility of Australian fatalities, the department said.

"In a disaster of this magnitude affecting a city where many thousands of Australians live and holiday, definitive advice on Australian casualties will not be possible for many days," a spokesperson said this morning.

"We can expect that reports may emerge of Australians affected by the quake over the next several days."

Australia has three consular officials on the ground in Christchurch, including its deputy high commissioner.

Seven staff, including the high commissioner, will be travelling to Christchurch from Wellington today.

Six DFAT staff from Canberra and four Centrelink social workers are flying to New Zealand.

At this stage, there are more than 1500 Australians registered as being in Christchurch and an estimated 8500 in the Canterbury region.

"We have confirmed the safety of more than 400 Australians and are seeking to confirm the status of about 1000 who are known or believed to be in the earthquake-affected area," the DFAT spokesperson said.

Queen's support

The earthquake in has left the Queen “utterly shocked”.

She described her feelings in a message of support sent to New Zealand Prime Minister John Key.

The Queen said in her brief statement: “I have been utterly shocked by the news of another earthquake in Christchurch.”

“Please convey my deep sympathy to the families and friends of those who have been killed; my thoughts are with all those who have been affected by this dreadful event.

“My thoughts are also with the emergency services and everyone who is assisting in the rescue efforts.”

US rescuers on their way

The State Department says a US rescue team is on its way to New Zealand to help with search and rescue.

Spokesman P.J. Crowley says Secretary of State Hillary Rodham Clinton phoned New Zealand Foreign Minister Murray McCully to express her sorrow over the earthquake. Crowley said in a tweet Tuesday that a US search-and-rescue team was en route.