A powerful 7.8-magnitude earthquake struck off the southwest coast of New Zealand on Wednesday, US seismologists said, prompting the issuing of a tsunami warning for the entire country. 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 33 kilometres, the US Geological Survey reported. The quake struck at 8:22 pm (1522 PST). The Hawaii-based Pacific Tsunami Warning Centre said: "An earthquake of this size has the potential to generate a destructive tsunami that can strike coastlines in the region near the epicentre within minutes to hours." The centre, whose warning extended to all of New Zealand, added it was not immediately known if a tsunami had been generated.