Deadly jellyfish stings are being touted as a possible cause of the mystery deaths of two French tourists snorkelling on the Great Barrier Reef.
Jacques Goron, 76, and Danielle Franck, 74, were found floating motionless in the water at Michaelmas Cay within minutes of each other while skin-diving on Wednesday morning.
Workplace Health and Safety are investigating the deaths, which happened in front of their relatives and other holidaymakers not long after the pair entered the water.
It's believed they suffered heart attacks, but Sydney cardiologist Dr Ross Walker has speculated an Irukandji jellyfish could be to blame because it was unlikely two people would die so close together.
"Irukandji are the size of your little fingernail, they're very small, you can't see them," he told media outlets.
"It's highly unlikely that two people are going to die within minutes of each other just because they've got underlying medical conditions."
The Irukandji is one of the world's most venomous creatures and Dr Walker said its toxin can take up to 20 minutes to send the body into cardiac arrest.
But dive company Passions of Paradise has played down the theory, saying it isn't the right time of year for Irukandji and Ms Franck had a full stinger suit on.
The two were not related and their next of kin, who were on the boat, are assisting with police inquiries.
Inspector Peter Mansfield said in Cairns police would prepare a report for the coroner, including what treatment was given to the pair.
Association of Marine Park Tourism Operators executive director Col McKenzie said Passions of Paradise had taken more than 400,000 tourists to the reef since if began operating in 1989.
North Queensland tourism operators don't believe the deaths will deter other visitors.
Max Shepherd, the chair of Tourism Tropical North Queensland, said tourism operators provided the highest level of duty of care to reef visitors.
Deputy Premier Jackie Trad expressed her condolences to the family and friends of the pair.
"I think it's important that we have a look at all of the factors that contributed to what actually happened," she said.