The majority of nearly 500 pilot whales found stranded off Australia’s remote southern coast has died, officials said on Wednesday, as rescuers struggled in freezing waters and fading light to free those still alive.

The group, which is the biggest beaching in the country’s modern history, were first spotted a wide sandbank during an aerial reconnaissance of rugged Macquarie Harbour in Tasmania state on Monday. Authorities had already been working to rescue survivors among an estimated 270 whales found Monday on a beach and two sand bars near the remote coastal town of Strahan on the southern island state of Tasmania.

Another 200 stranded whales were spotted from a helicopter on Wednesday less than 6 miles to the south, Tasmania Parks and Wildlife Service Manager Nic Deka said.

After two days of a difficult and dangerous rescue attempt, state marine scientists said at least 380 of the long-finned pilot whales had died.

By late Wednesday, around fifty of the mammals were freed but experts said there was a high likelihood they would return as many did during the rescue attempt a day earlier, creating an exhausting loop for rescuers who cannot work through the night.

The outlook for the remaining 30 stranded and still alive pilot whales, a species of oceanic dolphin that grow to 23 feet long and can weigh up to 3 tons, was bleak.

“As time goes on, they do become fatigued and their chance of survival reduces,” Mr. Deka said. “We do expect to rescue more but increasingly our focus is what do with the carcasses.”

Tasmania is the only part of Australia prone to mass strandings, although they occasionally occur on the Australian mainland.

Australia’s largest mass stranding had previously been 320 pilot whales near the Western Australia state town of Dunsborough in 1996.

Marine Conservation Program wildlife biologist Kris Carlyon said the latest mass stranding was the biggest in Australia “in terms of numbers stranded and died.”

Why the whales ran aground is a mystery. The pod may have been drawn into the coast to feed or by the misadventure of one or two whales, which led to the rest of the pod following, Mr. Carlyon said.

“It’s really likely this was the one stranding event of a big group. This would have been one big group offshore,” he said.

Marine scientist Vanessa Pirotta said there were a number of potential reasons why whales might become beached, including navigational errors.

“They do have a very strong social system, these animals are closely bonded and that’s why we have seen so many in this case unfortunately in this situation,” Ms. Pirotta said.

And rescuing them does not always work “because they are wanting to return back to the pod, they might hear the acoustics for the vocalizations of the sounds that the others are making, or they’re just disoriented and in this case extremely stressed, and just probably so fatigued that they in some cases don’t know where they are,” she added.

In neighboring New Zealand, more than 600 pilot whales washed up on the South Island at Farewell Spit in 2017, with more than 350 dying.

This article contains information from Reuters and The Associated Press.