Numerous people are trapped in buildings after a large 6.3 magnitude earthquake struck Christchurch, only five months after the city was badly damaged by a 7.1 magnitude temblor.

New Zealand's TV3 television aired rescue operations in the collapsed Pyne Gould Guinness building, where 200 people had reportedly been trapped.

The network said the provincial chambers building had also collapsed and people were believed to be trapped inside.

Witnesses said there were buildings down all around Cathedral Square in the city, with the church destroyed. Residents said the quake lasted about a minute.

"The shake has been a lot worse, maybe not in intensity but as far as damage is concerned, and there are numerous people trapped," a fire service spokesman told Radio New Zealand.

Hospitals around the South Island are being cleared to take the hundreds of casualties of this afternoon's devastating earthquake.

Police said fatalities had been reported at several locations and that two buses had been crushed by falling buildings.

Declaring a state of emergency, Christchurch mayor Bob Parker said he holds grave concerns for the safety of people in Banks Peninsula area.

"We still have yet to hear any reports in from Banks Peninsula and I'm very very concerned about that," he told Radio New Zealand.

Christchurch is located immediately north of the peninsula.

The quake occurred at 12.51pm in the central city. Power was out in 80 percent of the city, as large aftershocks continued.

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