Tiny, highly venomous marine stingers are likely to have caused the deaths of two French tourists on the Great Barrier Reef near Cairns, a leading cardiologist has said.
Danielle Franck, 74, and Jacques Goron, 76, both died while snorkelling at Michaelmas Cay, about 33km north east of Cairns, on Wednesday.
Their deaths had been attributed to heart attacks, with the tour operator saying both had pre-existing heart conditions and swallowed seawater.
However, leading Sydney cardiologist Ross Walker told news.com.au dual fatal heart attacks occurring within just moments of each other, attributable only to pre-existing medical conditions were highly unlikely.
He said the pair were likely to have been stung by the tiny, highly venomous Irukandji jellyfish, which cannot be seen in the water.
"Two divers were affected by swallowing seawater? Give me a break," he said.
"Look at the facts here. You have jellyfish infested water, and the bite of the jellyfish stimulates a heart attack."
"I think they were more likely all bitten by Irukandji jellyfish.
"But if that gets out, then the people running the dive programs won't get anyone in the water."
The pair were among a group of 21 French nationals who were on a day trip with Passions of Paradise tour company to the popular snorkelling site.
The tour company's chief executive Scott Garden said the deaths were the result of pre-existing conditions and a "perfect storm" of unlikely events, including sea water being swallowed.
But both Walker and tropical medicine expert Tarun Sen Gupta agreed the tiny, highly venomous Irukandji jellyfish is most likely to blame.
The thumb-sized stingers are found in tropical waters, usually between October through to May.
They are the smallest and most venomous box jellyfish in the world.
Walker said most people stung by the tiny marine creatures did not realise they had been bitten.
"This jellyfish is the size of a finger nail, and you can't see them in the water," he said.
"You don't even know you've been bitten straight away. Around 30 minutes later you have symptoms and go into cardiac arrest."
Franck was pulled unconscious from the water about 11am Wednesday.
Shortly after, the crew also saw Goron in distress.
He was pulled onto the beach.
CPR was performed on both tourists, including by a doctor who was on a nearby vessel.
However, the pair was unable to be revived.
Sen Gupta agreed with Walker's assessment.
The James Cook University medical education director described two people having heart attacks in the same place at the same time without being caused by an external factor was, "one in a million".
"I would think there may have been something in the water, some toxin, maybe an envenomation," he told the Cairns Post.
"I think you would have to look at that."
Walker said elderly people were far more likely to die from an Irukandji jellyfish sting than younger people.
"When an older person gets bitten, it causes all sorts of issues," he said.
"To have a cardiac arrest at the same time, and the other a short time later, either they (the tourist operator) didn't think of the Irukandji jellyfish sting, which is bizarre because signs are everywhere and they are in the water from November until March, or they simply don't want to scare people."
Association of Marine Park Tourism Operators executive director Col McKenzie said Passions of Paradise had an impeccable safety record.
It's only previous safety blemish, he said, was the death of an 80-year-old tourist in 1997.
"Accidents like this are a tragedy for the surviving family members, the crew and the passengers," he said.