Numerous buildings were damaged in the quake, and police reported some limited looting in the city [Reuters]

A strong 7.1 magnitude earthquake has struck New Zealand's South Island, 30 km west of Christchurch, damaging roads, buildings and severing electricity across the region.

No tsunami alert was issued but a state of emergency was declared in the area to facilitate recovery operations early on Saturday morning.

According to Radio New Zealand, just two people were seriously injured but authorities said significant damage had been done and police have had to deal with cases of looting in shops across the city of Christchurch.

Mike Coleman, a police inspector, told New Zealand's National Radio that it was "very unsafe to be out and about".

"There is considerable damage in the central city and we've also had reports of looting - just shop windows broken and easy picking of displays," he said.

'Significant damage'

According to Bob Parker, the city's mayor, 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.

Michelle Hider, the communications manager of the Christchurch hospital told Radio New Zealand that "many" people had suffered minor injuries, including cuts and and broken bones.  

"Two men in their fifties were seriously hurt, one is in ICU after being hit by a falling chimney, while the other was hurt by broken glass".

Alf Stewart, another police inspector, told the radio that some people had been arrested for looting.

"We have some reports of people smashing [storefront] windows and trying to grab some property that is not theirs ... we've got police on the streets and we're dealing with that," he said.

Angela Morgan, a Christchurch resident, told the Associated Press news agency that the fronts of at least five buildings in the city had collapsed.

"Roads have subsided where water mains have broken and a lot of people evacuated in panic from seaside areas for fear of a tsunami," she said.

Morgan added that there were reports that "some people were trapped in damaged houses".

Frequent quakes

Another person from Christchurch, Kevin O'Hanlon, said the jolt was extremely powerful.

"I was awake to go to work and then just heard this massive noise and 'boom,' it was like the house got hit. It just started shaking. I've never felt anything like it,'' he told the Stuff News website.

The earthquake hit at 4:35am on Saturday (1635 GMT Friday) and was about 33km below the Earth's surface, the geological agency GNS Science said.

The Pacific Tsunami Warning Center said "no destructive widespread tsunami threat existed, based on historical earthquake and tsunami data".

New Zealand sits above an area of the Earth's crust where two tectonic plates collide. The country records more than 14,000 earthquakes a year, but only about 150 are felt by residents. Fewer than 10 a year do any damage.