Twenty-one long-finned pilot whales have died but 11 were saved after beaching themselves at Bruny Island, south of the Tasmanian state capital Hobart, conservation officials said on Friday.

The whales became stranded on Thursday and were spotted by a passing yacht, which alerted authorities who rushed to Butlers Beach and frantically doused them with water hoping to keep them alive.

Overnight, using mats and boats, rescuers managed to move 11 of them back into the water.

"Eleven survived, 21 died. Most of them were dead when they were discovered', said Department of Primary Industries spokeswoman Rosemary Gales.

She added that because there was no road access to the secluded beach, the corpses would be left to decompose.

Whale strandings happen periodically in Tasmania, but scientists do not why, although Gales has her own explanation.

"Pilot whales are really social animals so if one makes a mistake and gets into shallow water, the others will come," she said.