Two French tourists who died within minutes of each other while snorkelling on the Great Barrier Reef are likely to have been stung by the Irukandji jellyfish, a cardiologist says.

The pair, aged 74 and 76, suffered heart attacks and died in the water at Michaelmas Cay, east of Cairns, on Wednesday.

The ABC understands one was wearing a stinger suit, one was not, however both tourists had pre-existing medical conditions.

Cardiologist Dr Ross Walker said it was highly unlikely two people would die in such quick succession, regardless of their health.

"I think it's highly likely they were stung by Irukandji," Dr Walker said.

"Irukandji are the size of your little fingernail, they're very small, you can't see them.

"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."

The Irukandji is one of the world's most venomous creatures.

Dr Walker said its toxin can take time to manifest, and can send the body into cardiac arrest.

The pair who died were among a group of 21 people who took the Passions of Paradise tour. ( Supplied: Passions of Paradise )

"The point is it doesn't happen as soon as you're stung, it can wait 15 or 20 minutes before it actually impacts on the body," Dr Walker said.

Simply speculation, says tourism boss

Col McKenzie from the Association of Marine Park Tourism Operators said the jellyfish claims were simply speculation.

He said crew members from the Passions of Paradise tour boat were in the water with their passengers.

"Environmental issues played no role in this whatsoever. Nobody else saw any jellyfish, nobody else got stung," Mr McKenzie said.

He said the crew handled the situation well.

"To have two separate incidents occur within 10 minutes of each other is unheard of," he said.

"I've had 35 years in the industry and I've never heard anything like that."

A Passions of Paradise boat at Michaelmas Cay, where two French tourists died. ( ABC News )

Reef 'absolutely safe'

Deputy Premier Jackie Trad expressed her condolences to the families of the tourists and those travelling with them.

She said it was important to consider all of the factors that contributed to the incident, but defended the region's safety record

"It is absolutely safe," she said.

"It is a world icon and we encourage lots of people to come to Queensland to look at the reef.

"We will have a look at all of the factors that contributed to these fatalities but right now our condolences and sympathies go to the friends and families."