Two French tourists have died from heart attacks while on the same snorkelling tour in waters north-east of Cairns. 

A third French tourist was originally believed to have also suffered a heart attack, but their condition was later downgraded to “shock”.

CEO of reef tour operator Passions of Paradise, Scotty Garden, said he had sympathy for the “elderly” pair, who both had “pre-existing medical conditions”.

The two deceased, a woman and a man, had been snorkelling with a group of 18 others on popular diving spot Michaelmas Cay when tragedy struck.

The group returns to Cairns after the incident:

Passions of Paradise has arrived back into Cairns after today’s reef tragedy which claimed two lives. pic.twitter.com/fUKN3TYe3q

— Cairns Post (@TheCairnsPost) November 16, 2016

Cairns police told a press conference the 74-year-old woman had been spotted floating face down by a staff member before being pulled onto the boat for treatment, while the 76-year-old man was dragged onto the beach for treatment.

A Cairns-based rescue helicopter was called, but was unavailable for unknown reasons.

Mr Garden said there had been two guides in the water with the tourists, plus one lookout on the shore and one on the stationary catamaran.

“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,” he said.

Two staff and a doctor from a nearby boat tried to resuscitate the pair, but were unsuccessful.

The doctor declared the pair had died before emergency services were able to reach them.

The tour comprised 21 elderly people, the company said. Photo: Passions of Paradise

Mr Garden said he was satisfied with how the Passions of Paradise guides had responded.

“It is very stressful for our staff but their priority has been with helping the group,” he said.

On Wednesday afternoon, Association of Marine Park Tourism Operators executive director Col McKenzie had originally told media all three tourists had suffered heart attacks.

“It was a perfect storm of a fatal scenario with three French tourists suffering heart attacks at the same time,’’ Mr McKenzie said.

He told AAP the deceased has been on “some pretty severe medications”.

He voiced concern at the rescue helicopter being “unavailable” amid the chaos.

Michaelmas Cay is a small, vegetated stretch of sand 48km from Cairns. Photo: POP

“We’ve been really concerned about the lack of the rescue helicopter over the last few months,” he told the ABC.

“The latest fatalities, the helicopter hasn’t been available.

“We have no idea why they’re not providing any support.

“We just get told the helicopter’s not available.”

Passions of Paradise has been operating since 1987 and has taken over 400,000 tourists snorkelling, diving and sightseeing on the Great Barrier Reef.

In 1997, an 80-year-old woman died on a tour, also from heart problems.