A 60-year-old British man has died while diving on the Great Barrier Reef, an Australian tourism company has said.

He is the third person to die on the reef in as many days.

The man, from England, was rescued from the sea bed at Agincourt Reef, near Port Douglas, after his breathing regulator was seen out of his mouth.

He was 15m below the surface when he was found and pulled onto the boat, said Col McKenzie, head of the Association of Marine Park Tourism Operators.

The man was given oxygen and a defibrillator was used, but without success.

 A doctor airlifted to a nearby helicopter pad continued the CPR efforts but the man was pronounced dead.

He was on his second dive of the day and was travelling with his wife, the Cairns Post reported.

"Accidents like this are a tragedy for the surviving family members, the crew and the passengers," said Col McKenzie, who added the boat had carried 230,000 divers without any fatalities. 

The excursion was operated by the Quicksilver tour company.

Two French tourists - Jacques Goron, 76, and Danielle Franck, 74 - also died on Wednesday after suffering heart attacks while snorkelling on Michaelmas Cay, near Cairns.

The pair were among a group of 21 people "with pre-existing medical conditions", said the Passions of Paradise tour operator.

A lookout on the beach spotted Mr Goron floating in the water and he was pulled onto the sand for CPR.

Moments later, Ms Franck was also seen face down in the water and hauled onto a catamaran.

Despite the use of bottled oxygen and a defibrillation equipment they could not be saved.

"It might just simply be a coincidence, but we may well find out that one person saw the other person being rescued and that caused the onset of the second emergency," said tourism boss Col McKenzie.

"We have about five deaths each year on the Great Barrier Reef but in 35 years in the industry, I have never seen two people die on the same tour."