Dive company Passions of Paradise said that the man and woman were among a group of 21 elderly French tourists on a tour at Michaelmas Cay on Wednesday.
Two snorkel guides were with the group, while there was also one lookout on the beach, and one on the boat.
The beach lookout spotted the 76-year-old man floating in the water and pulled him onto the sand, while the lookout on the boat pulled the woman, who was 74, onto the catamaran, the company said.
Col McKenzie, executive director of the Association of Marine Park Tourism Operators, said the pair suffered sudden medical issues.
“I just don’t know what it was; however, from what I was told, it would lead you to believe that they probably had heart attacks,” he told Associated Australian Press.
“I do know that the woman was on some pretty severe medications as well.”
Crew members tried to revive the pair, but they both died at the scene.
“They had pre-existing medical conditions and were accompanied by a guide when they were in the water,” said Scotty Garden, Passions of Paradise chief executive, in a statement.
A third French tourist also got into difficulties but survived.
McKenzie described it as a “perfect storm”.
“Crew on the Passions boat worked to save them all, including CPR and using oxygen, but tragically two are dead,” he told the Cairns Post.
“There was a doctor onboard the nearby Ocean Spirit who declared them dead.”
The company had taken more than 400,000 tourists to the reef since it began operating in 1989, McKenzie said. One other fatality had occurred during a tour in 1997, when an 80-year-old died from a heart problem.
Two French tourists die from heart attacks on Great Barrier Reef
16 November 2016
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Two French tourists have died after suffering suspected heart attacks while on a snorkelling trip north of Cairns.
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