AFP

At least 100 people remained trapped in the ruins of Christchurch on Tuesday night following a devastating earthquake which claimed at least 65 lives.

New Zealand Prime Minister John Key announced on Tuesday 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 Tuesday'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 on Tuesday 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 on Tuesday 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 passers-by 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 on Tuesday 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 latest damage 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 on Tuesday night, while a NSW search and rescue team was due to start work at first light on Wednesday.

In Canberra, Prime Minister Julia Gillard warned Australians could be among those killed or injured in the earthquake, but said no Australian casualties had been confirmed as of 1800 (AEDT).

Heavy aftershocks continued to roll across Christchurch on Tuesday night, further frightening shocked residents. A 5.0 magnitude jolt hit at 7.43pm (1743 AEDT).