Survivors were being still being pulled from the rubble of buildings in central Christchurch overnight on Tuesday, in the aftermath of a powerful earthquake that is known to have killed 65 people.

Police say the death toll will rise as they are still finding bodies, though they also say people are continuing to be recovered alive.

The 6.3-magnitude quake struck at 12.51pm on Tuesday, 10km south-east of the central city at a depth of 5km. Scores of strong aftershocks have been felt and a state of emergency has been declared for at least the next five days.

Hundreds of search and rescue personnel, fire service staff, army personnel and volunteers have been working under floodlights through the night to get to those who are still trapped.

Superintendent Dave Cliff says the scene is grim, but over 100 people have been freed so far and another 100 are still thought to be trapped. People trapped in buildings have been texting for help.

Roads, buildings, homes and core infrastructure have been extensively damaged. Civil Defence says cordons have been set up around the one-way system in the central city.

Deputy Prime Minister Bill English says people must leave central Christchurch because it is not safe. Police are urging people to stay at home and check on their neighbours, while schools in the greater area are closed until further notice.

Hundreds of people have spent the night at welfare centres for quake victims. Centres have been established at Burnside School and Hagley Park North, and a third, at Addington Raceway could not take any more people on Tuesday night.

Christchurch is still recovering from a 7.1-magnitude quake that hit the Canterbury region on 4 September last year, causing widespread damage but no deaths.

Earlier in February, survivors gathered for the 80th anniversary of the 7.8-magnitude earthquake in Napier, in which at least 256 people died.

The number for information about emergency centres is 0800 779 997

The Red Cross call centre for people worried about friends and family in Christchurch is 0800 RED CROSS, or 0800 733276.

Buildings collapse

The Pyne Gould building in Cambridge Terrace has collapsed and fire fighters are trying to free trapped people.

22 people have been rescued alive from the building so far and police were talking to others still trapped inside as they worked to free them overnight.

The Canterbury Television Building has also collapsed and was on fire on Tuesday night, while the spire of the historic Christchurch Cathedral crumbled when the quake struck.

Power supplies were cut to 70% of Christchurch. Lines company Orion said about 150,000 households were without power and it was impossible to say when supplies would be restored.

Civil Defence says there is no water supply to 80% of the city.

Search and rescue teams in city

Search and rescue teams have arrived from other parts of the country, and an Australian Hercules with 150 people is expected with an urban rescue team early on Wednesday.

About 200 extra police officers have been drafted in from throughout the country. The Defence Force says it has approximately 200 soldiers in Christchurch helping officers maintain cordons around the city.

Fire fighters were arriving from Timaru to help overworked fire crews, who said they could not cope with the volume of quake-related callouts.

Army medical teams are working at four different locations, while Defence Force fire fighters are also working with local fire brigades.

An Air Force plane carrying 20 St John Ambulance officers and sniffer dogs was due to arrive in Christchuch late on Tuesday night.

Two Iroquois helicopters have arrived to help with rescue efforts, while an Orion is taking aerial photographs of affected areas.

Three Navy vessels, including the Canterbury and Resolution are in Lyttelton Harbour and sailors are assisting rescue efforts. A fourth navy vessel is on its way from Oamaru.

State of emergency declared

Christchurch Mayor Bob Parker says a state of emergency has been declared. The damage in the city is "immense" and the city will need help from other parts of New Zealand.

Mr Parker says the central city has been cordoned off and it was not safe for people to go into the area. Search and rescue personnel would be visiting "dozens" of buildings overnight on Tuesday.

Many buildings have also collapsed in Lyttelton. Chief fire officer Mark Barclay says 60% of the buildings in the main road, London Street, have fallen.

Hospital functioning

Christchurch Hospital remains open and large numbers of seriously injured people are being treated in the emergency department.

Triage centres have been set up at Latimer Square, the Sanitarium Factory in Papanui, the Beasley Avenue 24-hour Medical Centre, Princess Margaret Hospital and Christchurch Hospital.

Some 70 army medical staff have been deployed and will be based at the central police headquarters.

Wellington Regional Hospital and hospitals throughout the South Island are discharging patients and cancelling non-urgent surgery to free up beds for quake victims.

City roads damaged

Local roads have sustained major damage and there are reports of liquefaction and surface flooding. However, main roads to Christchurch are open.

The New Zealand Transport Agency said the Lyttleton Tunnel had been reopened on Tuesday for emergency vehicle use only.

The Ferrymead bridge is reported to be impassable because of rising water.

However, the Rakaia Bridge on SH1 south of Christchurch and the Waimakariri Bridge on SH1 north of the city are open, the Transport Agency says.

Christchurch Airport is closed to all but emergency flights.

Use cellphones for emergencies only

Police say the emergency 111 service is working, although callers are having problems getting through due to network damage, congestion and overloaded lines.

Trapped or injured people should keep trying 111, police said.

Civil Defence and Telecom are asking people to make only emergency calls so that injured or trapped people can get through to emergency services.