6 September 2010 Last updated at 23:48 ET

Quake-hit Christchurch hit by strong aftershocks

Residents have been busy clearing up following Saturday's powerful earthquake

Violent aftershocks have rattled the New Zealand city of Christchurch following a powerful earthquake that caused widespread damage.

More than a dozen aftershocks struck overnight - two measuring a magnitude of 5.4, officials said.

Mayor Bob Parker said the tremors were further weakening buildings damaged in Saturday's 7.0-magnitude quake.

Almost two-thirds of the city's 160,000 homes were said to have been damaged, although there have been no fatalities.

A state of emergency has been extended until Wednesday, and the city centre remains cordoned off.

Experts warned that more tremors were likely.

"It is still possible that we will have a magnitude six in the next week," said Ken Gledhill, a monitor at the geological agency GNS Science.

"People ought to be aware of that, particularly if they are around structures which are already damaged. For a shallow earthquake like this, they will go on for weeks," he added.

Some of the city's most historic buildings are among those having to be pulled down because they are beyond repair.

Engineers are also checking fresh cracks in the city's Christ Church Cathedral, local media reported.

New Zealand's civil defence ministry said power had been restored to most of the city and all major roads and rail links were open.

Contamination risk

About 300 people left homeless by the quake have been sheltering in welfare centres. Residents are also being advised to boil water following the risk of contamination from burst sewage pipes.

Prime Minister John Key has warned that New Zealand's economic recovery will suffer because of the earthquake.

New Zealand lies at the southern end of the so-called Pacific Ring of Fire, and above an area of the Earth's crust where the Pacific Plate converges with the Indo-Australian Plate.

The country experiences more than 14,000 earthquakes a year, of which only about 20 have a magnitude in excess of 5.0.

The last fatal earthquake was in 1968, when a 7.1-magnitude tremor killed three people on the South Island's western coast.