Wellington, Sept 4 NZPA - Civil Defence has declared a state of emergency following a massive earthquake near Christchurch this morning.

The magnitude 7.1 earthquake struck about 30km west of Christchurch at 4.35am today, at a depth of 10-12km, causing injuries, structural damage and gas leaks, cutting power, and rupturing water and sewerage mains.

Roads are blocked by debris, some in the central city caused by building frontages of older buildings collapsing. Cars have been crushed under the rubble, and wardens are warning people to stay away from older buildings as they fear further collapses.

Civil Defence Minister John Carter rushed to Wellington, where the National Crisis Management Centre in the Beehive has been activated to monitor the quake, and co-ordinate central government response.

He announced a state of emergency had been declared about 10am, and said it would give Civil Defence officials more power.

Mr Carter, Prime Minister John Key and Energy Minister Gerry Brownlee would fly to Christchurch aboard an airforce plane, arriving about 2pm.

The United Nations had contacted the Government and was being kept informed of the situation, which Mr Carter described as a "significant disaster".

"Stay calm," was Mr Carter's message to Christchurch residents.

"I know it's a difficult thing and people are in shock, and that's understandable.

"It's a pretty horrible situation."

Plans were in place to get milk tankers full of water into the city but the water was likely to be rationed and people must conserve it, as it was likely to be several days before it was restored.

Mr Carter said many people were suffering from shock but he urged the public not to flood the hospital with minor injuries. They should instead go to their local 24-hour medical centres.

"The most important thing we can tell anybody at the moment is please don't panic," he said.

"Be safe, make sure your friends and family are safe."

The army had been informed and was ready to mobilise if needed, and all systems were operating as they should in an emergency, Mr Carter said.

Mr Brownlee, who is MP for Ilam, said he expected power to be restored this afternoon.