Up to 90 whales died after becoming stranded in a remote bay at the Australian island state of Tasmania.
On Tuesday, rescuers said the mission to save another 180 still stuck would be a challenge.
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 and dozens of whales manoeuvring for space.
Rescuers could be seen wading in the water tas they attempted to refloat the whales in deeper areas.
Government marine biologist Kris Carlyon said "about a third" of the 270 animals were dead by late Monday and rescuing survivors would 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 can only be reached by boat, limiting the number of rescuers who can reach them.
About 60 people, including volunteers and 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," Mr Carlyon said from the nearby town of Strahan.
However, Mr 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 rescue, which was affected by 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.
Mr Carlyon said rescuers would still have to triage the whales, prioritising the healthiest and easiest to reach.
Most of a group of 30 on a nearby beach were found dead on Monday, while about 60 other whales on the sandbars are also believed to have died.
Mr Carlyon said that once the whales are returned to the water, the biggest challenge would be herding them out of the sandbar-riddled harbour and back into the open ocean.
Mr Carlyon suggested the pod may have gone off track after feeding close to the shoreline or by following one or two whales that strayed.
Karen Stockin, an expert in marine mammals at New Zealand's Massey University, said Tasmania was a whale strandings were common in the area.
"It seems to be a notorious whale trap. You do tend to get these mass stranding events there," she told AFP.
Stockin said that while pilot whales were typically more resilient than other species, rescuers faced a race against the clock as the mammals can overheat, their muscles deteriorate and their organs become crushed outside their natural environment.
Their highly social nature was also expected to work against the rescue effort, she said, with some freed whales likely to re-strand themselves to remain close to the pods.
"Time is never your friend. So without doubt the more expedited rescue missions are, the more likely there is an increased chance of survival," she said.