Dozens of pilot whales have died after becoming stranded on a sandbar on Tasmania's West Coast.
Rescuers have been desperately trying to save the pod of 270 whales who became became stuck on Monday morning.
Tasmania Parks and Wildlife Service's Nic Deka said progress has been slow, with conditions and the half-submerged whales making rescue efforts tricky.
This morning's phase will be critical in determining what is possible,' he told reporters on Tuesday.
'Basically we'll take the animals with the best chance to start with and the ones that we are able to deal with.
'Some animals may be simply too big or in an unsuitable location.'
Mr Deka said about one third of the whales had already died as of Monday evening and most were inaccessible by boat.
'In terms of mass strandings in Tasmania, this is the trickiest we've had to deal with,' he added, saying the mission could take days.
About 60 people are helping with the rescue, including 40 parks and wildlife staff and personnel from nearby fish farms, which have supplied boats.
Mr Deka said pilot whales were a robust species and the survivors have a chance of lasting several days on the sandbars if the weather stays cool.
'It's ugly for people on the ground but as far as the whales go, it's ideal,' he said.
The whales got into trouble on Monday morning but the rescue couldn't begin until marine specialists were able to survey the scene.
Mr Deka said multiple rescue methods would be trialled and a lot would depend on how the whales respond.
It is understood to be the biggest mass stranding in Tasmania in more than a decade.
