A tourist snorkelling off Australia's pristine northwest coast was airlifted to hospital in a serious condition after being struck by a whale, medics said Monday.

The victim was exploring near the World Heritage-listed Ningaloo Reef in the Indian Ocean when she was struck by the tail of one of the giant creatures. St John Ambulance service told AFP they transferred the woman to a remote airport for evacuation and emergency treatment in Perth.

She sustained "chest injuries" and was said to be in a "serious but stable condition," after suffering a "crush from being hit by a whale's tail," a spokesperson said.

Other reports said the 29-year-old was struck by a whale shark, the world's largest fish which can reach a length of 15 meters (50 feet).

The area is well known for its close encounters with whale sharks and the even larger humpbacks. Both species are normally cautious around boats and humans.