There are about 250 whales stranded at Macquarie Harbour on the West Coast, more than double the earlier estimate.
Marine conservation experts are working to rescue the pod, which was originally thought to be about 70 whales.
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The Department of Primary Industries, Parks, Water and Environment reported the stranding about 11.30am on Monday and provided the larger estimate at 4.30pm.
The stranded animals, believed to be pilot whales, are believed to be on a sandbar and stranded inside Macquarie Heads.
There are also approximately 30 whales reportedly beached at Strahan's Ocean Beach.
Ben Bramich from the West Coast said it was "really sad" to see about 30 whales about three to five metres long beached at Ocean Beach.
"At Ocean Beach there were about 30 whales beached and a lot of them had already died, I reckon it would be about 90 per cent," he said.
"This is the most I have ever seen, in 2011 I saw beached whales but not as many as there are there now."
Mr Bramich and Carmen Wilson saw the whales on the beach at 11.30am on Monday and alerted authorities who were attending the pod of whales stranded on the sandbar.
"They weren't there last night when we were at the beach so it seems to have happened overnight," Mr Bramich said.
"We hope they can get in and get as many as they can out before it gets to a worse state."
DPIPWE staff and Tasmania Police are on at Macquarie Harbour and managing the incident.
"Marine experts will assess the scene and the situation and will provide updates as the day progresses," the department said in a statement.