Multiple fatalities have been confirmed after a powerful 6.3-magnitude earthquake struck the New Zealand city of Christchurch, bringing down buildings and buckling roads.

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.

Police said Central City was being evacuated, with multiple deaths reported at several locations, including two buses crushed by falling buildings.

Quake has caused multiple deaths

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

Second major quake to hit city in six 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

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

Police said there were reports of multiple building collapses, fires in buildings and the sounds of screams from people trapped inside. They said Christchurch Hospital has not been evacuated and was in operation.

Local television reports that bodies have been pulled from a local youth hostel.

Three triage centres have been set up in Central City, Sydenham and Papanui, and police are advising those who have evacuated their homes or buildings to report to their closest Civil Defence sector post.

Unconfirmed reports suggested there were no more ambulances available and that police had taken to evacuating those injured in their squad cars.

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Eyewitnesses described the scene as "horrendous" and authorities in New Zealand have confirmed that there have been multiple fatalities.

Witnesses say people have been trapped by buildings brought down by the earthquake, and television pictures showed cars buried under piles of rubble and injured people being helped along the street.

One woman said it 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 NZ Herald was reporting a man and 19 of his colleagues are currently 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."

ABC journalist Emma Pollard is in Christchurch and said the quake lasted for around 30 seconds to a minute.

She described hearing glass smashing and people screaming outside and said most of the historic buildings in the central part of Christchurch she walked through had been reduced to rubble.

New Zealand correspondent Dominique Schwartz said the quake brought down facades and buildings, and created flooding in main streets due to burst water mains.

Authorities said 80 per cent of Christchurch was without power, and emergency phone lines are also severely impaired.

Prime Minister John Key addressed Parliament in Wellington and said details were still "extremely sketchy".

"The people of Canterbury are once again going through a frightening experience," he said.

"The fear is that this quake has taken place at a time when Cantabrians are going about their business in a very populated time.

"I cannot rule out whether there have been fatalities or not."

Mr Key said the government was aware of significant damage to buildings that had people in them at the time, and promised to keep New Zealanders informed as information becomes available.

He said he would fly to Christchurch, conditions permitting, and has called an emergency cabinet meeting for 3:00pm.

Mr Key said communication with Christchurch officials was limited, and asked those able to leave the CBD to do so immediately.

'Dreadful aftershock'

Christchurch mayor Bob Parker says the earthquake felt at least as big as the large one which hit the city in September.

"A dreadful, dreadful aftershock, it felt more like a whole new earthquake to me," he said.

"It was very, very strong. I was up on the top floor of the council building. I got thrown quite a distance and I know of people in our building that have been injured and I've had some unconfirmed reports of some fairly serious injuries."

Christchurch Cathedral in the city square - already badly damaged in the quake last year - has suffered further damage and looked like completely collapsing.

"It is huge. We just don't know if there are people under this rubble," a priest standing outside the rubble of a damaged cathedral told New Zealand television.

On one main arterial road close to the city centre there is reportedly a huge hole in the ground which a truck has fallen into.

There were also reports of children at St Mary's primary school in the middle of the city bunkering down in the middle of a playground.

Many people are now out on the streets, and TV NZ is reporting that the Christchurch police station has been evacuated.

Twitter users have reported feeling the quake in Wellington and Dunedin.

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.

- ABC/wires