Two French tourists have simultaneously suffered fatal heart attacks, while a third is being treated for shock at a popular dive and snorkelling spot on Australia's Great Barrier Reef.

Tourism company Passions of Paradise confirmed to media on Wednesday the man and woman, both in their 70s who tragically died were part of a group of 21 French tourists snorkelling at Michaelmans Cay, 45 kilometers northwest of Cairns in Queensland state earlier in the morning.

"They had pre-existing medical conditions and were accompanied by a guide when they were snorkelling in the water," Passions of Paradise chief executive Scotty Garden said in a statement.

The lookout on the beach spotted the man floating in the water, and pulled him onto the sandy Cay, while a lookout on the catamaran pulled the woman on-board. Staff attempted to resuscitate the pair, but were unsuccessful, Garden said.

"A third person was treated for shock by staff on board the boat," Garden said. Its believed that victim has been transferred to Cairns base hospital.

Michaelmas Cay is a world-famous bird sanctuary that provides a rare opportunity to both snorkeling and diving from a sandy beach in calm, protected waters.

Police aren't treating the deaths as suspicious.

Passions of Paradise has extended its sympathies to the family and friends of the victims.