Quake rocks N Zealand South Island

But despite the severity of Friday's quake there were just two reports of serious injuries and prime minister John Key, who flew to Christchurch to inspect the damage, said it was "an absolute miracle" that no-one had died.

The quake, which hit 19 miles west of Christchurch, according to the state geological agency GNS Science, shook a wide area, with some saying buildings had collapsed and power had been cut. No tsunami alert was issued.

Christchurch mayor Bob Parker declared a state of emergency four hours after the quake rocked the region, warning people that continuing aftershocks could cause masonry to fall from damaged buildings. The emergency meant parts of the city would be closed off and some buildings closed as unsafe, he said.

Minister of civil defence John Carter said a state of civil emergency was declared as the quake was "a significant disaster" and army troops were on standby to assist. He said the "sharp, vicious earthquake has caused significant damage in parts of the city ... with walls collapsed that have fallen into the streets".

Chimneys and walls had fallen from older buildings, with roads blocked, traffic lights out and power, gas and water supplies disrupted, he said.

"The fronts of at least five buildings in the central city have collapsed and rubble is strewn across many roads," Christchurch resident Angela Morgan said. "Roads have subsided where water mains have broken and a lot of people evacuated in panic from seaside areas for fear of a tsunami."

Christchurch fire service spokesman Mike Bowden said a number of people had been trapped in buildings by fallen chimneys and blocked entrances, but there were no reports of people pinned under rubble. Rescue teams were checking premises.

Christchurch Hospital said it had treated two men with serious injuries and a number of people with minor injuries. One man was hit by a falling chimney and was in intensive care, while a second was badly cut by glass, hospital spokeswoman Michele Hider said.

Looters broke into some damaged shops in Christchurch, police said.