A whale that became stranded on a sandbar in Tasmania's south will be left to decompose.

The stranding at South Arm was reported about half past one but the Bryde's whale died about six o'clock last night, despite the efforts of a marine conservation team to keep it cool.

Marine biologist Kris Carlyon said it is unusual for this type of whale to be found in Tasmania.

He says it may have had navigation problems in the shallows.

"We are almost 100 per cent sure that this is a Bryde's whale which is quite a rare species, we've only had a handful of records in Tasmanian waters, it prefers much warmer waters, around the tropics, not anything less than about 20 degrees so quite surprising to find this animal down in the south east of Tasmania," he said.