A geologist says that the powerful earthquake that struck a New Zealand city on Saturday morning appears to have torn a new fault line in the Earth's surface.

Mark Quigley, a geology professor at Canterbury University, said the 7.1-magnitude quake was caused by an ongoing collision between the Pacific and Tectonic plates.

He said the extensive damage that it wrought on the South Island city of Christchurch may have come as a possible "new fault" tore across the ground and pushed some surface areas up.

"One side of the earth has lurched to the right ... up to 11 feet (3.5 metres) and in some places been thrust up," Quigley told National Radio.

"The long linear fracture on the earth's surface does things like break apart houses, break apart roads. We went and saw two houses that were completely snapped in half by the earthquake," he said.

New Zealand Prime Minister John Key said it was a miracle that not a single fatality occurred in the city of 400,000 people, where at least 500 buildings were destroyed.

"If this had happened five hours earlier or five hours later (when many more people were in the city), there would have been absolute carnage in terms of human life," Key told TV One News on Sunday.

Essential services are still getting back up and running in the aftermath of the quake, but Christchurch Mayor Bob Parker said "our first priority is just people."

"That's our worry," Parker said.

As of Sunday, water supply had resumed for all but 15 to 20 per cent of city residents. Ninety per cent of the city had its power back. It is expected to take a long time to completely repair the city's water and sewage services.

The army took over security for the centre of Christchurch on Monday, and Parker extended a state of emergency there for an additional two days.

The centre of the city remained cordoned off. Building and business owners were the only people allowed to access the area.

State geologist John Ritsau said the city has seen a half-dozen aftershocks of magnitude 5.0 or greater, as well as 50-plus aftershocks that registered as 4.0 or greater.

"It's been a very long aftershock sequence so far," Ritsau told CTV News Channel by telephone from Wellington, New Zealand, on Sunday.

Ritsau said that Saturday's event occurred in a place that is not typically as prone to earthquakes as other parts of the country.

"New Zealand being a very earthquake-prone country, you can get large-magnitude earthquakes almost anywhere," he said.

"But this particular area of the country, the amount of seismic activity is actually quite low compared to a number of other areas in the country. So, this actually did come as a bit of a surprise that we had this earthquake where we did."

The last major earthquake in New Zealand occurred in July of last year, when a 7.8 magnitude quake rumbled South Island's Fiordland region.

With files from The Associated Press