Orcas were dubbed killer whales after ancient sightings of them ganging up against other (frequently larger) whale species. More recently, they have also been known to attack and kill great white sharks.
"You can see what looks like teeth marks and I just can't think of anything else that could have done this," a witness told British tabloid media. But experts say white sharks are not known in the region.
Swedish couple Emma Andersson and Billy Ludvigsson spoke of the drama when they saved the lives of the British backpackers attacked by a shark in the Whitsunday Islands off the northeastern Queensland coast.
"I have seen numerous sharks at Werri using drones, but this is the first time I have been able to warn anyone with the speaker system," said operator Christopher Joye, who released the footage online.