A key undersea cable that was destroyed in a volcano eruption in Tonga may take at least four weeks to be repaired, said New Zealand's foreign ministry.

The cable was severed during a volcanic eruption on Saturday, crippling communications and cutting the Pacific island off from the outside world.

The eruption also caused a tsunami, which left at least three dead - including a British national.

Teams have been working round the clock to get vital supplies to the region.

"US cable company SubCom advises it will take at least four weeks for Tonga's cable connection to be repaired," a statement by New Zealand's foreign affairs ministry said on Wednesday.

SubCom, a repair contractor for more than 50,000 km (31,070 miles) of cable in the South Pacific, said it was working with telecommunications company Tonga Cable to mobilise a ship for the cable repairs, Reuters reported.