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

The epicentre of the undersea quake was located about 160 kilometres 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 above normal.

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

Emergency management spokesman Vince Cholewa said initial reports indicated a wave about 17 centimetres in height reached southern port of Bluff about an hour after the quake.

"We're just waiting on confirmation of that, and we will cancel the warning for a potential tsunami," he said.

New Zealand's Institute of Geological and Nuclear Sciences (GNS) upgraded its reading to 7.8 after initially measuring the quake at 6.6