There has been a mass stranding of 20 sperm whales on Tasmania's west coast, with only four whales stuck in shallow water believed to still be alive.

The pod was discovered on Ocean Beach near Strahan early Saturday morning, but authorities say conditions in the water are too dangerous for rescuers to intervene.

Nearby, rescuers were making progress in freeing another eight sperm whales stranded on a sand bar in Macquarie Harbour, about four kilometres south of the beach.

Chris Arthur from the Parks and Wildlife Service says four of them were swimming freely, with a fifth joining them later in the afternoon.

With the help of fish farmers and jet boats, authorities managed to free the whales, but he says getting them to open ocean from the harbour entrance proved difficult.

"The weather conditions won't allow us to get them out so it's a matter of just managing them at the moment," he said.

Rescuers will resume their bid to save the whales at first light, but say it could take several days to free the ocean giants.

Authorities are urging people to stay clear of the channel between Hells Gate and Table Head.