There have been "multiple fatalities" after a shallow 6.3 magnitude earthquake in Christchurch this afternoon caused buildings to collapse, police have confirmed.
Police said fatalities had been reported at several locations and that two buses had been crushed by falling buildings.
Christchurch resident Jane Smith, who works in the central city, told the Herald a work colleague had just returned from helping rescue efforts after a building facade had collapsed on a bus on Colombo St.
"There's people dead. He was pulling them out of a bus. Colombo St is completely munted."
TV3 reported that a person had died in the Christchurch suburb of Sumner.
Police said there were reports of fires in buildings in the central city and of people being trapped.
Police said all available staff were helping with the rescue operation and the Defence Force had been called in to assist.
Triage centres have been established for the injured at Latimer Square in the central city, Spotlight Mall in Sydenham and Sanitarium in Papanui.
A Herald reporter said that emergency services were struggling to enter the central city and were having to manoeuvre slowly around gridlocked traffic.
Herald reporter Jarrod Booker said the shake lasted approximately one minute and was extremely violent - rocking buildings back and forth.
He said people had left buildings and were out on the streets where tarmac had cracked and water mains had burst, causing extensive flooding.
Tuam Street had become a river as water poured from ruptures in the road and was impassable in places.
The whole central city was in grid lock as people tried to evacuate central businesses to check their homes, Jarrod Booker said.
Most traffic lights were out and cars were also having to negotiate around hordes of people on foot.
Jarrod Booker said that he could hear sirens but that it would be difficult for emergency services to access the city because of the gridlock.
"Even sitting in a car you can feel continual shaking on a smaller scale than the original quake," he said.
Some pedestrians were standing on the footpaths and staring into space, apparently in shock.
Jarrod Booker said Christchurch's historic cathedral of the Blessed Sacrament on Barbadoes Street had half collapsed, with the remaining part of the building filled with cracks.
There was huge damage to other older buildings with large amounts of debris falling to the ground, he said.
He said the carpark at the Christchurch Star had turned into a river with huge cracks and that the roads had risen in areas.
People were comforting people outside amid a general state of shock as they tried to absorb what had happened, he said.
Radio New Zealand reported widespread damage to the city centre, with a church on Durham St collapsed and concrete lifted by up to a metre.
TV3 reported the Provincial Chambers Building had collapsed and it was believed people were trapped inside.
Speaking to media at the Beehive's National Crisis Centre, Director of Civil Defence John Hamilton said a response plan was now being put together using all available national resources.
"That includes extra fire people, extra police personnel, assets from the Defence Forces. International offers of assistance are coming through from Australia in particular."
Mr Hamilton said the earthquake was a level three crisis - the highest for a localised event.
Phone lines are down and calls are not being connected to emergency services. Telecom said it is working to understand which services have been affected by the earthquake and get these restored as soon as possible.
Flights into Christchurch have been put on hold while Christchurch Airport checks the state of its runway. So far, four international flights have been diverted to Wellington Airport.
Today's quake was shallower and closer to Christchurch than the original Darfield quake, which took place 30km west of the city at a depth of 33kms.
New Zealand Police announced a large-scale evacuation of the central city was under way on the agency's website. According to the news release, the earthquake killed an undetermined number of people at various locations around the city, including passengers on two buses crushed by buildings that had fallen on them.
Colombo St is completely munted