Up to 90 whales have died and 180 more are still stranded in a remote bay in southern Australia as a "challenging" rescue operation began on Tuesday.
Scientists said two large pods of long-finned pilot whales became stuck on sandbars in Macquarie Harbour, on Tasmania's rugged and sparsely populated west coast.
Images showed shallow water thick with scores of the large slick-black mammals manoeuvering for space.
Government marine biologist Kris Carlyon said "about a third" of the animals were dead by Monday and rescuing survivors would be a "challenging" task likely to take several days.
Though mass whale strandings occur relatively often in Tasmania, such a large group has not been seen in the area for more than a decade.
The animals are only accessible by boat, limiting the number of rescuers who can reach them.
About 60 people - including volunteers and local fish farm workers - are involved in the rescue attempt.
They are battling cold, wet conditions as well as the harbour's unusual tides which are dictated by barometric pressure.
"In terms of mass whale strandings in Tasmania, this is up there with the trickiest," Carlyon told reporters in the nearby town of Strahan.
However, Carlyon said many of the partially submerged whales should be able to survive for the several days it would take his team to complete the task, in part due to the inclement weather.
"It's pretty ugly for people on the ground but as far as the whales go its ideal - it's keeping them wet, it's keeping them cool," he said.
Carlyon said rescuers would still have to "triage" the whales, prioritising the healthiest and most accessible.
Most of a 30-strong group on a nearby beach were found dead Monday, while about 60 other whales on the sandbars are also believed to have since died.
Once the whales are returned to the water, Carlyon said, the biggest challenge will be herding the social creatures out of the sandbar-riddled harbour and back into the open ocean.
Scientists said it was unclear what caused the latest stranding, but have suggested the pod may have gone off track after feeding close to the shoreline or by following one or two whales that strayed.
The latest stranding comes as a humpback whale that was stuck in a tropical river in Australia's north finally returned to the ocean after more than two weeks.
Public broadcaster ABC reported the creature, which spent 17 days in the crocodile-infested waters of Kakadu National Park, has been spotted in open seas off Darwin.
Scientists had been weighing options for guiding the humpback to safety after it became the first known whale to travel up the muddy river, but were relieved when it returned to sea on its own.