WA Police say a male diver in his 50s was attacked around 10.30am (WST) on Tuesday, with possible serious injuries.
A Department of Fisheries spokeswoman said more information was being sought about the attack, which occurred about 180km east of Esperance.
A St John Ambulance spokesman said a volunteer ambulance brigade had been called from Condingup, about 68km from Esperance.
The spokesman said the 55-year-old man would be brought by boat to land then driven to Esperance Hospital, hopefully within two hours, and a helicopter had been provisionally ordered to bring the diver to Perth, depending on the severity of his injuries.
The area of coast they were diving off is only accessible by 4WDs.
The man is understood to have been diving with a friend, who called emergency services when the man was bitten by the shark.
He is conscious, the spokesman said.
A Surf Life Saving spokesman said Esperance’s coast was quite dangerous, with big sharks frequently seen in the area, while fishermen were often swept off rocks.