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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.
Shallow quake
GNS Science said today's quake was centred at Lyttelton at a depth of 5km at 12.51pm.
GNS said the earthquake would have caused more damage than the original 7.1 earthquake on September 4 because of its shallow depth.
Its data centre manager Kevin Fenaughty said residents said the quake's epicentre was located in the "worst possible location" for the city.
"It's a nightmare. A lot of people were just getting back on their feet after the original quake."
Another earthquake of 4.5 struck at 1.21pm, 10 km east of Diamond Harbour.
Streets flooded
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.
'Great confusion'
Mayor Bob Parker said he was "thrown quite a distance" by the earthquake.
"That was, in the city central anyway, as violent as the one that happened on the 4th of September," he told Radio New Zealand.
Mr Parker said there were scenes of "great confusion" on the streets, also saying the roads were jammed as vehicles sought to get out of the central city.
"I know of injuries in my building and there are unconfirmed reports of serious injuries in the city."
Mr Parker did not know the extent of damage to the city's infrastructure, but advised people not to drink the water supply.
"We've been through this before this once, we now need to think we did at that time."
Buildings collapsed
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.
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".
The spire on the Christchurch Cathedral had also collapsed.
A Newstalk ZB reporter in Christchurch said liquefaction was spewing out of the ground at St Albans High School.
School children had to be removed from the fields with liquefaction also spewing from the tennis courts.
Civil Defence response
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.
Civil Defence advice
The Civil Defence has issued the following advisory:
Check yourself first for injuries and get first aid if necessary before helping injured or trapped persons.
Assess your home or workplace for damage. If the building appears unsafe get everyone out. Use the stairs, not an elevator and when outside, watch out for fallen power lines or broken gas lines. Stay out of damaged areas.
Look for and extinguish small fires if it is safe to do so. Fire is a significant hazard following earthquakes.
Listen to the radio for updated emergency information and instructions.
Do not overload phone lines with non-emergency calls.
Help people who require special assistance - infants, elderly people, those without transportation, families who may need additional help, people with disabilities, and the people who care for them.
Detailed safety advice will come from local authorities and emergency services in the area. People should act on it promptly. MCDEM, local civil defence authorities and scientific advisors are closely monitoring the situation.
The Piko Wholefood building in Christchurch was badly damaged by the earthquake. 
A magnitude 6.3 earthquake struck the  of New Zealand at 12:51 PM local time on Tuesday (Monday 23:51 UTC). At least 75 people have been killed by collapsing buildings in central , with more feared. Mayor  said 55 bodies had been identified and there were a further 20 unidentified bodies. The spire of the iconic Anglican  has fallen and rubble is strewn throughout the central business district. Roads and carparks have cracked and lifted, and two buses are reported to be crushed under the bus exchange. Pools of mud have erupted due to burst water mains and liquefaction.  Boulders and falling cliff faces have destroyed buildings on hillside suburbs.  Fears for the safety of nearby towns Lyttelton and Akaroa are exacerbated due to communication problems.

The earthquake was centred near , at a depth of five kilometres, according to the . Unlike previous quakes in the region that caused no fatalities, Tuesday quake was shallower and closer to the central city and the damage was much worse. Condemned buildings, weakened by last year's widespread earthquakes, were destroyed. Some aftershocks have occurred in the area after the earthquake. The largest so far was a magnitude 5.6 which occurred at 7:04 p.m. February 21  (1:04:18 p.m. local time, February 22). 

Mayor Bob Parker has stated that as many as 25 major buildings in the city are destroyed. Urban Search and Rescue efforts are focussed on people trapped in the remains of Canterbury Television and Pyne Gould Corporation office buildings. The historic  in Christchurch has half collapsed, while the old Canterbury Provincial Chambers building, Piko Wholefoods, and a church on Durham Street have been destroyed.

The earthquake also caused an estimated 30 million tons of ice to break off of the Tasman Glacier forming icebergs in a lake near its foot. Tourists on boats at the time of the quake say waves of 3.5 meters swept the lake for at least 30 minutes following the event. The glacier sits on the country's west coast, approximately 120 miles (200km) from Christchurch. No injuries were reported.

Many people are trapped in damaged buildings or under rubble, but emergency services have been hampered by gridlock as motorists and pedestrians evacuated the CBD. The main hospital remains operational despite one damaged ward being closed, and three triage centres have been set up to provide medical aid. Several hundred delegates attending a medical conference in the city, the great majority from Australia, have been trapped in the city; some of these are assisting with tending to the injured.

Electricity, telephone services, and traffic lights suffered widespread outages. Orion and Telecom are attempting to assess the damage, and generators have been sent down from Auckland to replace the backup generators in the city. Civil Defence is mounting a response with all available national resources, and Cabinet is holding an emergency session. Speaking to ,   said he was "thrown quite a distance", that there were scenes of "great confusion" on the streets, and that the quake was "as violent as the one that happened on the 4th of September". The emergency telephone code, 111 was not working for the entire region of  but is apparently stable as of approx. 4 pm . Christchurch Airport is currently closed to all but emergency flights. Speaking after the earthquake, Bob Parker said at least 200 people are believed trapped under rubble, saying that New Zealand are "going to be presented with statistics that are going to be bleak".