A state of emergency has been declared in Christchurch, New Zealand, after a large 7.0 magnitude earthquake rocked the country's second largest city this morning.

The quake was felt throughout much of the South Island and southern parts of the North Island, and has caused widespread damage, including the collapse of some buildings.

It has also knocked out power and disrupted water and sewerage services.

Many people with injuries have sought treatment but hospitals say no-one has been killed.

Radio New Zealand reports Christchurch Hospital has treated two people seriously injured by falling masonry and glass, a number of people with broken bones, and others with minor injuries such as cuts and bruises.

The quake, which struck at 4:35am with the epicentre just 30 kilometres from Christchurch, is reported to have lasted for up to 40 seconds.

Christchurch Mayor Bob Parker told Radio New Zealand there was a lot of damage in the central city area, which has now been closed off for safety.

The director of civil defence at the National Crisis Management Centre, John Hamilton, is advising people to stay indoors and away from debris.

"If you're at home, stay home and make sure that your family and yourself are safe from further damage because there have been a number of aftershocks," he said.

"Limit use of telephones, keep off the roads and make every means you possible can to conserve water because the water and the sewerage system is broken."

Mr Hamilton says debris from buildings has closed some roads in central Christchurch.

"Most of the roads are open, there are some areas that have building debris on the roads and police have cordoned off the central business district, not because the roads are unusable but simply to make it safe or to ensure people aren't endangered by falling debris."

Roads in coastal suburbs were packed with cars as residents moved inland, but no tsunami alerts were issued.

Christchurch airport has also been closed.

Police Inspector Mike Coleman told Radio New Zealand says the city's central business district has been closed off for safety.

"Stay off the roads and also avoid the central city, there's considerable damage there and there's also been some looting," he said.

Journalist Tim Dower is in Christchurch and says there is a lot of rubble in the streets.

"We've had a number of reports of facades of buildings collapsing, many stone buildings in that part of the country and a lot of those have come down," he said.

"Some people have rung to say their water cylinders have split. The pipes have come away and there's flooding in their home."

The assistant editor of The Press newspaper in Christchurch, Colin Espiner, has told ABC Radio the quake is one of the strongest to ever hit the South Island.

"We're told structural damage to buildings, bridges are damaged, some roads that've been ripped up, that sort of thing," he said.

"Plenty of frightened people. The power is out to most of the city."

He says the quake was felt along New Zealand's east coast.

"There's been reports of damage and of the earthquake being felt from Dunedin right up to Wellington," he said.

"Also on the east coast, between the two cities, also felt it quite strongly, and as I say, right up to Wellington.

"But because it was centred so close to Christchurch that's been where the major damage has been occuring."

Local resident Paula May says it has been quite scary.

"We were upstairs in a wooden, old 1920s home and it was shaking," she said.

"I've got three small children. It was the scariest thing. We've got stuff down all over our house but we're alive."

Geoscience Australia senior seismologist Clive James says powerful aftershocks of 5.4 have been felt.

He says the cause of the earthquake is the junction of two plates.

"It's the junction of the Pacific and the Australian plates that goes all along New Zealand there, this is the manifestation of that," he said.