Screams can be heard from people trapped inside the scores of buildings destroyed by a 6.3-magnitude earthquake that has rocked the New Zealand city of Christchurch.

Prime minister John Key says at least 65 people have died. The city's emergency services are being stretched to the limit as they begin a widespread rescue and recovery operation.

Details of the overall devastation remain sketchy, but dozens of buildings have collapsed or been severely damaged, while many roads have buckled and streets are flooded by burst water mains.

The US Geological Survey said the quake struck near the city at a shallow depth of just four kilometres shortly before 12:50pm (local time) with the city at its busiest.

Television pictures show bloodied people being helped along city streets by rescuers.

Police said central city was being evacuated, with deaths reported at several locations including two buses crushed by falling buildings and the implosion of a local youth hostel.

Hospitals across the South Island are clearing patients to make room for the injured.

At least 17 confirmed dead

At least 17 confirmed dead Quake has caused multiple deaths

Quake has caused multiple deaths Second major quake to hit city in five months

Second major quake to hit city in five months Extensive damage in city, power cuts

Extensive damage in city, power cuts City has run out of ambulances

City has run out of ambulances 5.6-magnitude aftershocks recorded

5.6-magnitude aftershocks recorded A level 3 emergency has been declared

A level 3 emergency has been declared Contact DFAT on 1300 555 135

Christchurch Hospital has not been evacuated and is in operation, while three triage centres have been set up in Central City, Sydenham and Papanui.

The city's gas lines are being shut off to prevent further leaks and police are advising those who have evacuated their homes or buildings to report to their closest civil defence sector post.

Reports suggest there are no more ambulances available and that police had taken to evacuating those injured in their squad cars.

A number of buildings in the city centre are ablaze. Helicopters are dumping water on the burning CTV building and emergency crews are trying to pull down walls to access the fire.

Rescue teams are roaming the streets using sniffer dogs to locate survivors and bodies.

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Eyewitnesses described the scene as "horrendous".

Jeff McLay told TV NZ he was trapped with four other people when the floors of the multi-level Pyne Gould Guinness pancaked on each other.

About 200 people were working in the office block, although some may have left the building for lunch at the time the quake hit.

Mr McLay says the group sheltered in a space near a column which had compressed from three metres in height to about 1.5 metres.

"We were all thrown to the ground. When that happened, I thought, well, this is it," he said.

He says he was trapped for about 90 minutes before he was rescued.

Reporters said screams were heard inside the building and 30 people are believed to be trapped, however there has been one confirmed death.

Local television also showed vision of a rock that had fallen and flattened the Royal New Zealand Returned and Services' Association (RSA) building.

The Provinicial Chambers building, the Press newspaper office, and part of the CTV building have also reportedly collapsed.

The NZ Herald was reporting a man and 19 of his colleagues are trapped in the Forsyth Barr building on Colombo Street.

Gary Moore told the Herald that workers were stuck on the 12th floor after the building's stairwell collapsed.

"We watched the cathedral collapse out our window while we were holding onto the walls," he said.

"Every aftershock sends us rushing under the desks. It's very unnerving but we can clearly see there are other priorities out the window.

"There has been a lot of damage and I guess people are attending to that before they come and get us."

One woman said the earthquake was the scariest thing she has ever been through.

"It was awful. It was the most frightening thing I've ever experienced. My husband wasn't in the house, he was outside and he had been thrown into the vegetable garden," she told Radio New Zealand.

The iconic Christchurch Cathedral has also crumbled.

'Building just exploded'

Christchurch was struck by a major 7.1-magnitude earthquake in September last year. That quake was felt through much of the South Island and caused widespread damage.

The region has been struck by thousands of aftershocks since the original quake.

New Zealand, which sits between the Pacific and Indo-Australian tectonic plates, records on average more than 14,000 earthquakes a year, of which about 20 would normally top magnitude 5.0.

One resident in Lyttelton near the epicentre of the quake says the area looks like a bomb went off.

"It was extremely violent. I was sitting there with a friend and the building just like exploded. It was like a movie - it took two or three seconds to comprehend what was going on," he told Radio New Zealand.

"We ran onto the street and the front fell out of the building right in front of me.

"It looks like Lyttelton has been bombed... that wouldn't be an exaggeration. Everybody is out on the street and it was so violent that people are just dumbstruck.

"All the older buildings in Lyttelton, the fronts have fallen out of them and the army have sealed it up, so people don't walk near what is next to them.

"Everybody is talking to each other on the street which is kind of a nice feeling... there is nothing else to do. You just have to cope with it. I don't think anyone will be doing much sleeping for a while though."

Aftershocks

Aftershocks continue to rock the city, one of which sent people fleeing from the central square.

John Ristel from GNS Science in Wellington said there would be more aftershocks to come.

"There will definitely be aftershocks to this earthquake. An earthquake like this is big enough to basically generate its own aftershock sequence," he said.

"So we'd expect the aftershock activity to pick up again after this event."

Christchurch city councillor Barry Corbett says he thinks the damage from today's quake will be worse than last September.

"The streets are ripping up and the pipes that are underneath are gorging out quite a lot of not very nice water. So I have a feeling that this one might be worse than the first one on the fourth of September," he said.

Burst water mains have flooded the streets and 80 per cent of Christchurch is without power. Emergency phone lines are also severely impaired.

Prime minister John Key has promised to keep New Zealanders informed as information becomes available.

He says he will fly to Christchurch, conditions permitting, and has called an emergency cabinet meeting.

- ABC/wires