However, crews were able to save 25 of the animals on Tuesday and are aiming to escort more back into the sea.
The pilot whales were discovered in shallow waters off the west coast of the island on Monday.
It's unknown what drew the whales to the shore. Marine biologists say the rescue mission will likely take days.
Whale beachings are common in the region, but one of this size has not been seen in over a decade.
Tasmania last recorded a mass stranding in 2009 involving around 200 whales.