A pod of around 30 pilot whales became stranded on Bruny Island, south of the Tasmanian state capital Hobart, on Thursday, wildlife authorities said.

Department of Parks and Wildlife spokeswoman Liz Wren told the Hobart Mercury newspaper that 12 of the whales were still alive with people on the beach trying to move them back into the water.

"Preliminary reports indicate around 30 whales have stranded with some believed to be still alive," a statement from the department said.

Initial reports indicated they were pilot whales.

Whale strandings happen periodically in Tasmania, but scientists do not know why they happen.