New Zealand prime minister John Key has confirmed that at least 65 people are dead after a major earthquake devastated Christchurch on what he described as the country's "darkest day".

Thousands of shocked people are wandering the rubble-strewn city, many searching for loved ones and trying to reach trapped people.

Local television showed bodies being pulled out of rubble strewn around the city centre, while other footage showed onlookers clinging to each other and others bleeding and limping.

Screams rang out across the southern New Zealand city's main square as parts of Christchurch Cathedral toppled to the ground.

Mr Key said New Zealand may be witnessing "its darkest day".

At least 65 confirmed dead

At least 65 confirmed dead 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 Australia sends search and rescue teams

Australia sends search and rescue teams Contact DFAT on 1300 555 135

"The death toll I have at the moment is 65 and that may rise," he told TVNZ.

"It's an absolute tragedy for this city, for New Zealand, for the people that we care so much about.

"People are just sitting on the side of the road, their heads in their hands. This is a community that is absolutely in agony."

Police say central city is being evacuated. Two buses were crushed by falling buildings and a local youth hostel imploded.

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

Christchurch Hospital is in operation and three triage centres have been set up in central city, Sydenham and Papanui.

The strongest tremor, which was measured at a magnitude of 6.3, struck at a shallow depth of just four kilometres, at 12:51pm local time.

There had been two smaller tremors in the morning, and in the hours that followed there were 12 aftershocks, measuring up to 5.9 in magnitude.

It is the second major quake to hit the city in six months.

Live television pictures show several collapsed multi-storey buildings, while several are on fire.

Among those badly damaged include the Provincial Chambers building, the Press newspaper office and part of the CTV building.

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

Australians concerned about the wellbeing of friends and relatives have been urged to try to make contact with them directly.

A 24-hour consular emergency hotline, 1300 555 135, has also been set up.

Around 750 Australians are registered to be in Christchurch and there are as many as 8,000 in the Canterbury region.

Pip Ramby, who was rescued from the CTV building, says the building crumpled beneath her.

"We were in a meeting when the quake started, and it wasn't long before it was phenomenally disorientating," she told Radio New Zealand.

"There were about 10 of us on the fifth floor and we couldn't get out anywhere.

"It was hard to tell what had happened, but it wasn't long before we realised we were close to the ground and the roof had fallen in and people on the street just came to help us, came to pull us out of the wreckage.

"It was incredibly brave given all the structure was unstable.

"There was a baby in the room, so we got her lifted out first. Two of our staff were injured badly and they were taken to hospital. One of the women was in a critical condition... one of our administrators hasn't been accounted for yet."

Pancaked

Up to 30 people are believed to be trapped in the Pyne Gould building, where four storeys appeared pancaked on top of each other.

One woman was trapped on the top level of the building before being rescued by a crane.

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.

He 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.

Rescues are underway in many other buildings.

People have gathered in the middle of parks across the city, seeking safety from the danger of aftershocks and comfort in the company of others.

Roads across Christchurch are buckled and ground water is flowing across streets.

Civil defence and emergency management minister John Carter says authorities are working quickly to respond to the overwhelming need for help.

"Obviously we will give all the support that is needed. We will have all the agencies functioning," he said.

"We will be able to provide those from around the country to support and to fill those gaps, but immediately people need to make sure they are safe.

"They need to make sure their families are safe, that their neighbours - particularly elderly neighbours are safe."

Australian search and rescue personnel and medical teams will be sent to Christchurch and both the Government and Opposition have vowed to do whatever possible to help the rescue effort.

Foreign Minister Kevin Rudd said he was horrified at the TV footage of the earthquake says the Australia will "throw everything" at the disaster effort.

Mr Rudd says the Australian Government is sparing no expense, citing he would rather send too much support than not enough.

"The bottom line of this Government is, this is our Kiwi mates, so we pull out all stops," he said.

"This is a large-scale disaster and we need to act as one."

'Surreal' scenes

Australian man Simon Arms was competing in a seniors' tennis championships and said the initial quake lasted for about 30 or 40 seconds.

"It was like lightning ... thunder being right on top of you and no-one able to stand up," he said.

"The ground shook so hard that you had to literally fall over or hug the ground to stay safe."

Christchurch resident Alistair Dumbleton described the scene on the ground.

"There are roofs which are have been shaken to pieces, so all the tiles off the roofs are completely gone," he said.

"A train has been derailed, cracks in driveways, roads being ripped apart in the flash flooding.

"It's quite unreal, it's surreal. You have to pinch yourself."

Dr Marni Basto of Melbourne's St Vincent's Hospital was among dozens of doctors attending a medical conference in the city.

She says they have all been helping to treat the injured.

"For the time being we're just trying to accumulate any medical supplies that we can," she said.

"We've taken some from the stores around and everyone's been very helpful, giving medical supplies, everything they've got so far."

- ABC/wires