A powerful earthquake struck off the southwest coast of New Zealand this morning, prompting a brief tsunami warning but no casualties or major damage.

The epicentre of the undersea quake was located about 160km west of the city of Invercargill, on South Island, at a depth of around 35km.

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 did not pose a threat.

The Japanese meteorological agency put the preliminary magnitude at 7.8, while the Institute of Geological and Nuclear Sciences said it measured 6.6.

It said the tremor struck at 9.22pm local time.