Strong aftershocks are continuing to shake Christchurch as police confirm "multiple fatalities" after a 6.3 magnitude quake hit the city this afternoon.
Police said fatalities had been reported at several locations and that two buses had been crushed by falling buildings.
GNS Science said today's 12.51pm quake was centred at Lyttelton at a depth of 5km.
It was followed by a 5.7 magnitude aftershock at 1.04pm at a depth of 6km, 10km south of Christchurch.
The latest aftershock measured 5.5 and was the sixth recorded since the 6.3 earthquake. It was also recorded within 5km of Lyttelton at a depth of 5km.
Herald reporter Jarrod Booker said queues of cars could be seen being shaken up and down when the latest aftershock hit.
Jarrod Booker said cars stuck in the city's gridlock were being rocked side to side and occupants could be heard screaming.
"Even sitting in a car you can feel continual shaking on a smaller scale than the original quake," he earlier said.
Fatalities and buildings collapsed
Christchurch resident Jane Smith, who works in the central city, earlier told the Herald a work colleague had helped with rescue efforts after a building facade collapsed on a bus on Colombo St.
"There's people dead. He was pulling them out of a bus. Colombo St is completely munted," she said.
TV3 reported that a person had died in the Christchurch suburb of Sumner.
The broadcaster showed footage of people being rescued from the Pyne Guinness Gould building, where it is believed some 200 people have been trapped.
It said the Provincial Chambers Building had also collapsed and more people were likely also trapped there.
A listener told Newstalk ZB that the Piko Wholefoods building on Kilmore Street near the city centre, which was hit in the September 4 earthquake, was now "practically non-existent".
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.
The spire on the Christchurch Cathedral has also collapsed.
Christchurch resident Gary Moore told NZPA he and 19 other colleagues were trapped in Christchurch's Forsyth Barr building on Colombo Street.
Mr Moore said workers were stuck on the 12th floor as the stairwell had collapsed. He was not sure if people were trapped on other floors.
Emergency services have been struggling to access the central city and were having to manoeuvre slowly around gridlocked traffic.
Jarrod Booker said 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, he said.
Most traffic lights were out and cars were also having to negotiate around hordes of people on foot.
Some pedestrians were standing on the footpaths and staring into space, apparently in shock.
Jarrod Booker said the southern suburbs appeared to be particularly badly hit.
Liquefaction was forcing tarmac up in the middle of the road and water and sand were spewing out of chasms.
Civil Defence response
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.
Civil Defence Minister John Carter said all the South Island hospitals apart from Invercargill had been emptied to make way for earthquake victims.
Mr Carter and preparations had been made for a state of emergency to be declared.
He said the number of fatalities and the extent of the damage was still unclear.
Speaking to media at the Beehive's National Crisis Centre, Director of Civil Defence John Hamilton earlier 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.
Temporary accommodation is being organised for those who have been displaced, with tents possibly to be erected in Hagley Park.
All but emergency flights into Christchurch Airport have been put on hold while it checks the state of its runway.
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