An elderly French couple who died while snorkelling on the Great Barrier Reef could have died after being stung by a highly venomous jellyfish, a cardiologist says.
The couple, aged 74 and 78, were snorkelling at Michaelmas Cay near Cairns on Wednesday when their bodies were discovered floating in the water.
It was reported both had pre-existing medical conditions that may have contributed to their deaths.
Dr Ross Walker told ABC News it was highly unlikely two people would have died from heart attacks at the same time, as has been reported.
He suggested the tiny and deadly Irukandji jellyfish may have caused their deaths.
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"Irukandji are the size of your little fingernail, they're very small, you can't see them," he said.
"Let's look at fact and probability.
"It's highly unlikely that two people are going to die within minutes of each other just because they've got underlying medical conditions."
Dr Walker said it could take up to 15 to 20 minutes for the jellyfish venom to affect the body once someone was stung.
Col McKenzie from the Association of Marine Park Tourism Operators and Queensland Deputy Premier Jackie Trad have dismissed Dr Walker's claims.
Mr McKenzie labelled them speculation saying no-one else had seen jellyfish on the day and no-one else had been stung.
Ms Trad said the reef was "absolutely safe" and said all factors contributing to the incident would be examined.
