A PAIR of French tourists are dead after suffering heart attacks on the Great Barrier Reef while snorkelling.

An elderly man and woman, believed to be in their 70s, were snorkelling alongside each other while on a trip to Michaelmas Cay with the Passions of Paradise reef tour company. The popular reef site is 33km east of Cairns.

Passions of Paradise Chief Executive Officer, Scotty Garden, said in a statement the pair had pre-existing medical conditions and were accompanied by a guide before tragedy struck in the water.

A third person also suffered a medical condition, but survived the incident. They were part of a group of 21 elderly French travellers on board the catamaran, which has been operating on the reef for almost 30 years.

Gardens said the tour boat had a lookout on the beach, another on the boat and two snorkel guides in the water at the time, and that ‘all staff have current qualifications in CPR and first aid.’

“The Passions of Paradise lookout on the beach noticed a man floating in the water and pulled him to the sand cay where he performed CPR,” Garden said.

“He was then assisted by a doctor on the sand cay. The Passions of Paradise lookout on the boat saw a woman floating in the water and pulled her on to the catamaran where CPR was performed.”

It’s believed a Cairns-based rescue helicopter was not available at the time of the emergency.

A spokesperson from the Cairns Hospital said they had not received any more patients as a result of today's events and "have no indication that we will be receiving one.”

Executive Director of the Association of Marine Park Tourism Operators, Col McKenzie, told Australian Associated Press the pair had declared their serious medical conditions before entering the water.

“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 AAP.

“I do know that the woman was on some pretty severe medications as well.”