WELLINGTON - A powerful 7.8-magnitude earthquake struck off the southwest coast of New Zealand on Wednesday, the US Geological Survey said, prompting a brief tsunami warning but no casualties or major damage.

The epicentre of the undersea quake was located about 160 kilometres (100 miles) west of the city of Invercargill, on South Island, at a depth of about 35 kilometres, the US Geological Survey reported. It struck at 0922 GMT.

The Hawaii-based Pacific Tsunami Warning Centre said a low-level tsunami was generated near the epicentre, but later cancelled its warning, saying the waves no longer posed a threat to the whole country.

"Sea level readings confirm that a tsunami was generated. It may have been destructive along coasts near the earthquake epicentre," the Centre said, but noted sea levels were only about 20 centimetres (eight inches) above normal.

New Zealand civil defence officials said they had issued only a "potential tsunami" warning for the Southland area.

"We've had big differences in the measurements of the quake... we're issuing a precautionary message," said emergency management spokesman Vince Cholewa.

An aftershock struck roughly 20 minutes after the initial quake, seismologists said.

The USGS reported a magnitude of 5.8 for the aftershock, while New Zealand's Institute of Geological and Nuclear Sciences (GNS) put it at 6.1.

Radio New Zealand said that residents hundreds of kilometres away from the epicentre reported items falling off shelves, but said there were no casualties or serious damage thus far.

One resident of Invercargill told the New Zealand Press Association that the first tremor lasted at least a minute.

"Things just started to rattle a bit, then the house started to sway," said the man, who added that he, his wife and their three young children huddled under the dining table until the quake subsided.

Cracks had appeared around several door frames in the house, he said.

Invercargill police Inspector Olaf Jensen said there were no immediate reports of damage in the city, but the quake was significant enough to send staff running into doorways for safety.

Earthquakes are frequent in New Zealand, where the boundaries of continental plates meet as part of the Pacific "Ring of Fire," causing earthquakes and volcanic activity.