Two French tourists who died while snorkelling on the Great Barrier reef are thought to have been stung by lrukandji jellyfish.

The pair, aged 74 and 76, died within minutes of each other.

Cardiologist Dr Ross Walker said it was highly unlikely two people would die in such quick succession, the ABC reports .

“I think it’s highly likely they were stung by Irukandji,” Dr Walker told the ABC.

“Irukandji are the size of your little fingernail, they’re very small, you can’t see them.”

It is understood one of the victims was wearing a stinger suit and the other was not.

Both had pre-existing medical conditions.

Toxins from Irukandji can take minutes before impacting the body and sending it into cardiac arrest.

“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.

A group of 21 French tourists were on board the catamaran run by Passions of Paradise when the incident occurred.

In a statement, the company's Chief Executive Officer, Scotty Garden, said a third person also suffered a medical condition but survived, initial reports suggest they suffered a heart attack.

"They had pre-existing medical conditions and were accompanied by a guide when they were in the water," Mr Garden said.

The man was found floating in the water by a lookout on the beach, who pulled him onto the sand cay where he performed CPR, he was then assisted by a doctor.

The woman was also found floating in the water and was pulled onto the catamaran by a member of staff. She was unable to be resuscitated.

The company said the group were accompanied by a guide when they were in the water and lookouts on the beach and boat were on duty.

Michaelmas Cay. (Jake Smith / Wikimedia Commons)

Two snorkel guides were also with the group at the time. All staff had current qualifications in CPR and first aid, the statement said.

No causes of death have been confirmed by authorities, with the Association of Martine Park Tourism Operators saying the jellyfish claims were just speculation.