A powerful 7.0-magnitude earthquake has rocked much of New Zealand's South Island, but no immediate tsunami alert has been issued, and no injuries have been reported.

The quake, which hit 30 kilometres west of the southern city of Christchurch, shook a wide area, with some residents saying buildings had collapsed and power was severed. Christchurch police reported road damage in parts of the city of 400,000 people, with a series of sharp aftershocks rocking the area.

Police officers cordoned off some streets where rubble was strewn about from the quake. "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. Resident Colleen Simpson said panicked residents ran into the street in their pyjamas. Some buildings had collapsed, there was no power, and the mobile telephone network had failed.

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 said.

Ottawa resident Martin Lee has been in phone contact with his parents, who live in Christchurch. They've had things fall off shelves, but suffered no real damage to their home, he said.

They report roads in the city have massive holes in them. Water mains are out in many places in the city, and most of Christchurch is without power.

Lee said his parents are fine — feeling a mixture of excitement and nerves. The area is still experiencing magnitude 5 aftershocks.

Geological agency GNS Science said the earthquake was 33 kilometres below the Earth's surface.

New Zealand's National Radio reports the temblor hit at 4:35 a.m. local time, shaking thousands of residents awake.

There are no immediate reports of serious damage.