Wellington  - A powerful earthquake shook parts of New Zealand's South Island Wednesday, prompting a tsunami warning, scientists said.

The US Geological Survey measured the offshore quake at a magnitude of 7.8 earthquake, while New Zealand monitors put it at 6.6 magnitude.

"An earthquake of this size has the potential to generate a destructive tsunami that can strike coastlines in the region near the epicenter within minutes to hours," the Pacific Tsunami Warning Center in Hawaii said in an e-mail alert.

The quake was centred 90 kilometres south-west of the lakeside resort town Te Anau in Fiordland, but was felt 300 kilometres away in Dunedin, Radio New Zealand said.

It shook items off the shelves of supermarkets in Queenstown, toppled ornaments and cracked walls, but there were no immediate reports of serious damage.

The strongest known quake in New Zealand occurred in 1855 at an estimated 8.2 magnitude.

The country's biggest earthquake since instrumental recording began was the 1931 magnitude 7.8 Hawke's Bay earthquake. (dpa)