Irukandji jellyfish stings are likely to have caused the deaths of two French tourists, medical experts say.

Danielle Franck, 74, and Jacques Goron, 76, were pulled from the waters of the Great Barrier Reef within five minutes of each other on Wednesday morning, each having suffered a heart attack that proved fatal while on a guided snorkelling tour.

But a cardiologist, a tropical health expert and a marine biology expert have all agreed it was highly unlikely the dual cardiac events were coincidental.

All believe something in the water could have triggered the heart attacks.

Both Mrs Franck and Mr Goron had pre-existing health conditions but medical experts have said it is highly unlikely it was sheer coincidence the pair suffered fatal heart attacks in the same place and within minutes of each other.

Sydney cardiologist Ross Walker told news.com.au he believed the pair could have suffered stings from the tiny, highly venomous Irukandji jellyfish, which cannot be seen in the water.

The French nationals both died while snorkelling with a tour group at Michaelmas Cay, about 33km north east of Cairns, with respected tourist operator Passions of Paradise.

On Thursday afternoon, the company’s managing director Alan Wallish, maintained the deaths were a pure, tragic coincidence.

“We don’t get Irukandjis at this time of year and the two parties were very separate,” he said.

“The lady was 15 metres away from the boat ... (and) had a full stinger suit on.

“I really can’t see how jellyfish or stingers would have contributed in any way.”

However, Dr Walker said dual fatal heart attacks occurring within just moments of each other, attributable only to pre-existing medical conditions and the possible swallowing of sea water were highly unlikely.

“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.”

Townsville Irukandji jellyfish expert Professor Mike Kingsford, the head of James Cook University’s marine and tropical biology school, said there had been just two recorded fatalities from more than 200 stings by the tiny creatures.

He said one was a person with a pre-existing heart condition and the other a child.

Passengers disembark the Passions Of Paradise boat as it arrives back at the Cairns Marina after two French tourists died at Michaelmas Cay on Wednesday. Picture: Anna RogersSource:News Corp Australia

“At the end of the day, there are multiple possibilities,” he said.

“Previously, of the 200 people stung by Irukandji, less than one per cent have died, so it’s very unusual to die.

“Up there, I would have thought the people would be wearing stinger suits at this time of year.”

Despite that, however, Prof Kingsford said an Irukandji sting remained a possibility in the cause of the French tourists’ deaths.

But Mr Wallish said the strain of snorkelling could have contributed to the deaths.

“I think these days you have an older demographic travelling and they want to experience life, they don’t want to stand on the boat and look over the side and look at fish,” he said.

“Snorkelling on the reef can be strenuous … (particularly) if you have a medical condition.”

The pair were among a group of 21 French nationals who were on a day trip to the popular snorkelling site.

Passions of Paradise chief executive Scott Garden said on Wednesday the deaths were the result of pre-existing conditions and a “perfect storm” of unlikely events.

But while Dr Walker said the tiny, highly venomous Irukandji jellyfish is most likely to blame, tropical medicine expert Tarun Sen Gupta remained open to other potential causes.

Like Dr Walker, however, 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”.

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.

Dr Walker said most people stung by the tiny marine creatures did not realise they had been attacked.

“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.”

Two French tourists died while snorkelling at the popular Michaelmas Cay.Source:Supplied

Mrs Franck was pulled unconscious from the water about 11am Wednesday.

Shortly after, the crew also saw Mr 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, they were unable to be revived.

Mr Wallish said the company had suspended operations on Thursday in order for staff to receive counselling.

“In deference to the crew and families we’ve stopped today, we have a psychologist talking to staff about trauma ... It’s been a very traumatic and shattering experience for them,” he said.

He said all passengers on the tour had been briefed about the dangers of snorkelling and Mrs Franck and Mr Goron had both filled medical forms out in French.

He said he did not believe there should be tighter regulation of snorkelling.

“Now this generation is retiring and has the money and freedom to travel they want to experience things,” he said.

“With any adventure activity there is always slight risk and yesterday was possibly an example where that risk can happen.”

However, like Dr Walker, Dr Sen Gupta speculated there must have been something in the water that prompted the fatal cardiac arrests.

“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.”

Dr Walker said elderly people were far more likely to die from an Irukandji jellyfish sting than younger people.

Passengers disembark the Passions Of Paradise boat as it arrives back at the Cairns Marina. Picture: Anna RogersSource:News Corp Australia

“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.

Post-mortem examinations performed as part of a coronial investigation into the tragedy will determine their exact causes of death.

kim.stephens@news.com.au