Police Inspector Alf Stewart told the radio that some people had been arrested for looting.
"We have some reports of people smashing (storefront) windows and trying to grab some property that is not theirs ... we've got police on the streets and we're dealing with that," he said.
Christchurch Hospital said it had treated two men with serious injuries and a number of people with minor injuries caused in the quake.
One was hit by a falling chimney and was in a serious condition in intensive care, while a second was seriously hurt after being cut by glass, hospital spokeswoman Michele Hider said.
Christchurch Mayor Bob Parker said the "sharp, vicious earthquake has caused significant damage in parts of the city ... with walls collapsed that have fallen into the streets."
Chimneys and walls had fallen from older buildings, with roads blocked, traffic lights out and power, gas and water supplies disrupted, he said.
"The fronts of at least five buildings in the central city have collapsed and rubble is strewn across many roads," Christchurch resident Angela Morgan told The Associated Press.
"Roads have subsided where water mains have broken and a lot of people evacuated in panic from seaside areas for fear of a tsunami," she said, adding that "there is quite significant damage, really, with reports that some people were trapped in damaged houses."
Suburban dweller Mark O'Connell said his house was full of smashed glass, food tossed from shelves, with sets of drawers, TVs and computers tipped over.
"She was a beauty, we were thrown from wall to wall as we tried to escape down the stairs to get to safety," he told the AP. "It was pitch black (with the power cut) and we walked through smashed glass everywhere on the floor."
The quake hit at 4:35 a.m. (1635 GMT) shaking thousands of residents awake, New Zealand's National Radio reported.
Resident Colleen Simpson said panicked residents ran into the street in their pajamas. Some buildings had collapsed, there was no power, and the mobile telephone network had failed.
"Oh my God. There is a row of shops completely demolished right in front of me," Simpson told the Stuff news Web site.
Another person from Christchurch, Kevin O'Hanlon, said the jolt was extremely powerful.
"I was awake to go to work and then just heard this massive noise and 'boom,' it was like the house got hit. It just started shaking. I've never felt anything like it," he told the news Web site.
Christchurch International Airport was closed after the quake as a precaution, as experts prepared to check the runways and terminal buildings, a spokesman said.
The earthquake had a preliminary magnitude of 7.4 and was centered 6 miles (10 kilometers) below the Earth's surface, GNS Science said in a revised report, after initially stating it had hit 21 miles (33 kilometers) down. It noted that the shallowness of the quake meant greater damage across a wider area from the temblor.
The U.S. Geological Survey, in America, measured the quake at 7.0.
Radio reports said items were tossed from store shelves and roof tiles cracked by the strong temblor.
The Pacific Tsunami Warning Center said "no destructive widespread tsunami threat existed, based on historical earthquake and tsunami data."
New Zealand sits above an area of the Earth's crust where two tectonic plates collide. The country records more than 14,000 earthquakes a year -- but only about 150 are felt by residents. Fewer than 10 a year do any damage.