MULTIPLE deaths were reported as a 6.3 magnitude earthquake struck Christchurch toppling buildings, igniting fires and sending panicked people rushing into the streets.
The widespread damage came from a shallow lunch-time tremor in a city of 340,000 still recovering from a powerful quake in September, which did not cause any fatalities but did weaken many buildings.
Christchurch airport was temporarily closed and police said they were evacuating the city centre as building frontages collapsed, with witnesses saying there were people trapped inside.
Cars were buried under rubble and roads buckled as the tremor opened ruptures in the ground.
``It was just unbelievable, it was so strong, nothing like I've ever experienced before, just horrible,'' local shopkeeper Julian Hogday told New Zealand's TV3.
Police feared multiple deaths, including in two buses that were crushed by falling debris.
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``Multiple fatalities have been reported at several locations in the central city, including two buses crushed by falling buildings. A doctor and emergency services are attending,'' a police statement said.
``Other reports include multiple building collapses, fires in buildings in the central (city) and persons reported trapped in buildings.''
New Zealand Prime Minister John Key feared the worst after the shallow quake struck at 12.51pm (1051 AEDT).
``It is a very populated time with people at work, children at school,'' he said.
``Sadly I cannot rule out whether there have been fatalities or not, but we are aware of significant damage to buildings that had people in them at the time.''
On September 4, Christchurch suffered the most destructive quake to hit New Zealand in 80 years when a 7.0-magnitude tremor damaged 100,000 homes, leaving a clean-up bill estimated at $NZ4.0 billion ($A3.03 billion).
The city remained under a state of emergency for weeks with police cordoning off the centre for fear of collapsing buildings, as thousands of aftershocks hit the region.
Historic Cathedral destroyed
A large number of buildings have been destroyed by the quake, including a historic cathedral in the central city.
Twenty trapped in city building
Twenty people are trapped on the 12th floor of a Christchurch building.
One of those trapped, Gary Moore, says he and 19 other colleagues are stuck on the 12th floor because the stairwell collapsed in Tuesday's earthquake.
People were in a state of shock but were not injured, he told NZPA. He was not sure if people were trapped on other floors of the Forsyth Barr building, in central Christchurch.
Moore described the quake, which was followed by several aftershocks, as catastrophic.
``We watched the cathedral collapse out our window while we were holding onto the walls.''
``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,'' he said.
Meanwhile, television pictures showed firefighters rescuing people trapped in other buildings in the city centre, including a woman plucked from a building believed to be a hotel.
Christchurch Mayor 'thrown across room'
Christchurch Mayor Bob Parker told Radio New Zealand that he was thrown across the room by the quake, which some residents said shook for up to a minute.
Parker said there were scenes of ``great confusion'' on the streets, with roads jammed as people fled the central city.
``I know of injuries in my building, and there are unconfirmed reports of serious injuries in the city,'' Parker said.
New Zealand's Institute of Geological and Nuclear Sciences said the quake was centered near Christchurch at a depth of 5 kilometers at 12:51 p.m. local time and will have caused more damage than the original 7.1 earthquake on Sept. 4 because of its shallow depth.
The September quake, with an epicentre 30 kilometres west of the city at a depth of 33 kilometres, struck in the early morning when the central city was largely empty.
Today's quake brought down power lines and forced the evacuation of many buildings including the airport, university and hospital.
Communications into the area are difficult with phone systems disrupted.
Christchurch Hospital has not been evacuated and is in operation.
The Christchurch Police Station was initially evacuated but is operational again.
Twitter feed tells of quake horror
For many people social network site Twitter was among the earliest to provide on the scene reaction to today's big 6.3 magnitude Christchurch earthquake.
On Twitter people were providing instant reports on the quake.
One person tweeted: ``Holy f.... giant earthquake. city is in chaos! Buildings down everywhere.''
Another person said ``I now have no front or back doors. Shaking like a leaf.''
Another tweet added: ``In town.. Chaos. Cathedral came down. Came down! Getting out of the city.''
It's understood Lyttelton had also been badly hit, with buildings badly damaged, according to various tweets.
One tweet reported the quake felt huge on the fifth floor of the Commerce building with fallen concrete in stairwell.
8.58am
Sky News NZ quotes the civil defence minister as saying there are unconfirmed reports of fatalities. Several witnesses on the streets reported people injured in their offices when
the quake struck just after 1pm local time
8.56am Christchurch airport has been closed.
Earlier "You can see cars rocking as they go down the road," Sky News correspondent Kate King reports.
"I saw the ground role up in front of me. It lasted for about 20 to 30 seconds."
King said she saw fresh cracks appear in the road and "waterfalls of water" streaming as they pushed out of the ground.
"It's fair to say I'm a bit scared. There are people hugging each other. It's quite traumatic. Quite a lot of these people have lost a lot financially and mentally."
The shock measured 6.3 on the Richter scale.
Christchurch was rocked by a 7.1 earthquake in September last year causing widespread destruction.
This aftershock is the most severe of the thousands of aftershocks since then.
Local reports say buildings and houses had collapsed all around the city centre and the city's iconic cathedral had been destroyed.
Christchurch Hospital was being evacuated, a spokeswoman for the hospital said.
Witnesses have told local reporters there would be "deaths" this time.
Power was out in the city and phone lines were down.
"It was incredibly violent, very very scary," one visitor to the city, Philip Gregan told AAP over the phone.
"We're all standing out on the street with sirens going off around us.
"Oh no, there's another one,'' he said while on the phone.
"I want to get out of here."
Auckland GNS Science said the quake was centred at Lyttelton at a depth of 5km.
The New Zealand Herald reported that the jolt was felt in the capital Wellington to the north and the city of Dunedin to the south.
Residents say it lasted about a minute.
Tarmac on the road was cracked and water mains had burst, flooding the streets with water.
Christchurch was hit with a devastating magnitude 7.1 on September 4 last year.
The epicentre was 40km west of Christchurch at a depth of 10km.
The city, New Zealand's third largest, has been shaking regularly since, with aftershocks up to 5.1 magnitude recorded.
