An English tourist had his foot bitten off by a shark while another was bitten during the attack in the Whitsunday Islands near Australia's Great Barrier Reef, officials said.
In the latest in a string of shark attacks in the tourist area, a 28-year-old man's right foot was bitten off while a 22-year-old man suffered serious lacerations to his lower left leg, according to Mackay Base Hospital.
The pair were in a "serious but stable" condition in hospital after being airlifted from the resort town of Airlie Beach, an official told AFP.
They had been on a boat tour of the Whitsundays Islands when they were attacked while swimming in Hook Passage, according to the RACQ Central Queensland Rescue, which operated the helicopter flight.
They were "wrestling in the water" at the popular snorkelling spot when the attack occurred, an RACQ spokesperson told AFP.
"One of the patients was attacked first and the shark was believed to (have) come back" to attack the second, Queensland ambulance service operations manager Tracey Eastwick told reporters in Mackay.
Two foreign paramedics were on board the boat and provided initial first aid as the men were rushed back to the shore, Ms Eastwick said.
In the past year there have been several reported shark attacks in waters around the Whitsundays, which had been considered safe for swimming.
A woman and child were believed to have been mauled by a shark in January, while a man died of his injuries and a 12-year-old girl lost a leg in two separate attacks last year.
Despite tens of millions of visits to the beach in Australia annually, shark attacks remain rare.
There were 27 shark attacks in the country's waters in 2018, according to data compiled by Sydney's Taronga Zoo.