An overnight curfew has been imposed in New Zealand South Island's central Christchurch and Kaiapoi following the major earthquake that rocked the region early Saturday.

The 7.1-magnitude quake occurred at 4.35 a.m. local time on Saturday, 40 km west of Christchurch, at a depth of approximately 10 km. There have been 22 aftershocks, the strongest at 5.3- magnitude, New Zealand Institute of Geological and Nuclear Science reported.

The curfew was imposed from 7 p.m. on Saturday until 7 a.m. on Sunday local time.

Police were maintaining a cordon between Kilmore, Madras, Montreal and St Asaph streets. Building facades have collapsed, cars have been crushed and many homes have been damaged.

Police were urging people to stay at home and say anyone breaching the curfew can be arrested.

Civil Defence said that up to half a dozen homes have been left uninhabitable in the Selwyn district, and the township of Kaiapoi, also badly damaged in the quake, is also under curfew.

A state of emergency declared in the Selwyn district and Christchurch will remain in force in the city until at least midday on Monday.

Eighty police reinforcements from Auckland were expected to arrive in Christchurch on Saturday night to assist with recovery efforts, and Prime Minister John Key said the Army is likely to help with the clean-up too.

Key said the earthquake is New Zealand's worst natural disaster since the Inangahua earthquake in 1968.

Key arrived in Christchurch on Saturday afternoon to assess the extensive damage in the region following the 7.1-magnitude quake.

Key said it will take some time before the full cost of the quake is known but it may run into billions of dollars.

Meanwhile, Civil Defence Minister John Carter has advised people to keep calm, not to panic, and to be sensible.

The power lines company Orion says electricity has been restored to almost 90 percent of Christchurch but is proving harder to restore in rural areas.

The gas supplier to Christchurch, Contact Energy, says supply has not been affected by the earthquake, and gas is safe to use.

But in some areas, water supply have not restored.

Flights were resumed in Christchurch Airport on Saturday afternoon.

Two people seriously injured 

Two people have been seriously hurt. One man, in his 50s, is in a serious condition in Christchurch Hospital after being hit by a falling chimney, while another man was cut by glass.

The hospital says it is coping well with an influx of people. It has treated several people with broken bones and others with minor injuries such as cuts and bruises.

The Fire Service said search and rescue dogs have been used to search collapsed buildings, following reports that some people were trapped, but it says no one has been found so far.

Civil Defence headquarters has activated the national crisis center. No tsunami alert has been issued.