The Pacific Tsunami Warning Centre said there was a risk of a tsunami along coasts within 100km of the epicentre.

Shops 'demolished'

“I thought a large truck had just driven through the front window” - Susan Birkbeck Christchurch resident

New Zealand lies at the southern end of the so-called Pacific Ring of Fire, an arc of active and dormant volcanoes stretching along the eastern edge of Asia, north across the Aleutian Islands of Alaska, and south along the coast of North and South America.

The earthquake struck at 0435 on Saturday local time (1635 GMT on Friday), the USGS said, when most people would have been asleep.

The local newspaper, The Press, said it was felt widely across the South Island, including Christchurch and the nearby port city of Timaru, and that there were several aftershocks.

Police said damage and power outages had been reported as far afield as Dunedin, 360km (223miles) to the south-west, it added.

"There is a row of shops completely demolished right in front of me," Colleen Simpson told The Press, adding that many people in Christchurch was out in the street in their pyjamas looking scared and worried.

Susan Birkbeck, who lives in the centre of the city, said: "It was absolutely shocking, we're all terrified and scared of what's going to happen next."

"I was asleep when suddenly the house started shaking and there was this smashing sound, I thought a large truck had just driven through the front window."

"I'm now sitting on my bed surrounded by broken glass and I've no idea what to do. The walls and roof are just hanging, it's terrifying," she added.