In a puzzling tragedy off the Great Barrier Reef in Australia, two snorkelers died at the same time while swimming at the Michaelmas Cay on Wednesday, and a third snorkeler experienced "massive trouble" soon after, The Cairns Post and Daily Mail Australia reported.

“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,” Col McKenzie, executive director of the Association of Marine Park Tourism Operators, told Reuters.

"We're dumbfounded."

Upon disembarking the Passions of Paradise catamaran at Marlin Marina in Cairns around 6 p.m. local time, a woman witness told the media, "It was an awful day on the reef."

The victims were a 76-year-old man and a 74-year-old woman who were said to have preexisting medical conditions and died of heart attacks, according to the Associated Press.

In a statement to the media, Passions of Paradise chief executive Scotty Garden extended condolences to those involved and explained what happened at around 11 a.m.

The two victims were among a French group of 21 elderly people on the snorkeling tour aboard the catamaran.

The staff had a lookout on the beach, another on the boat and two snorkel guides in the water, and all have training 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.

"[Moments later] the Passions of Paradise lookout on the boat saw a woman floating in the water and pulled her onto the catamaran where CPR was performed.

"A third person later suffered a medical condition, but has survived."

The third snorkeler was believed to have suffered from an anxiety attack, not a heart attack as first believed, and was taken to a hospital.

"It was a perfect storm of a fatal scenario with three French tourists suffering heart attacks at the same time,'' McKenzie told the Post before learning more about the third victim.

"Crew on the Passions boat worked to save them all, including CPR and using oxygen, but tragically two are dead.

"There was a doctor on-board the nearby Ocean Spirit who declared them dead."

McKenzie later said the third victim "watched two of his travel companions in massive trouble, it would give anybody heart palpitations."

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From the Associated Press: "Passions of Paradise is an award-winning operation that has taken more than 400,000 tourists to the reef since 1989, according to McKenzie. The company's only previous fatality was an 80-year-old who died from a heart condition in 1997, he said.

"Police are preparing a report for a state coroner, who will determine the official causes of the deaths."

The incident comes on the heels of two snorkeling deaths off Far North Queensland last month. The Daily Mail Australia said a 67-year-old Japanese tourist died after snorkeling off Green Island, and a 23-year-old Korean tourist died while snorkeling at Lizard Island.

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