"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.

Further details were not immediately available.

The Great Barrier Reef stretches 2,000 km (1,200 miles) along Australia's northeast coast and is the world's largest living ecosystem.

It attracts more than two million visitors each year and brings in billions of dollars in tourism revenue.

"We have about five deaths each year on the Great Barrier Reef but in 35 years in the industry, I have never seen two people die on the same tour," said Col McKenzie, executive director of the Association of Marine Park Tourism Operators.