Eighty to 100 per cent of coral reefs surrounding Lizard Island off far north Queensland are now dead as a result of coral bleaching, scientists say.

What is coral bleaching? Occurs when abnormal environmental conditions cause coral to expel tiny photosynthetic algae, called zooxanthellae

Occurs when abnormal environmental conditions cause coral to expel tiny photosynthetic algae, called zooxanthellae Loss of colourful algae causes coral to turn white and "bleach"

Loss of colourful algae causes coral to turn white and "bleach" Bleached coral can recover if the temperature drops and zooxanthellae are able to recolonise them, otherwise it may die Source: ARC Centre of Excellence

The ARC Centre of Excellence for Coral Reef Studies at James Cook University visited 83 reefs stretching from Townsville to the Torres Strait earlier this year and revealed the worst recorded mass bleaching event in the reef's history.

Professor Andrew Baird said researchers conducted the same survey this month and had already visited the first 50 reefs between Townsville and Lizard Island.

He said the results were confronting.

"What we're seeing now is lots and lots of dead corals," he said.

"On most of the sites around Lizard Island between 80 to 100 per cent of corals are gone — there's not much coral at all, north of Port Douglas".

Professor Baird said there was a slim chance the northern third of the Great Barrier Reef could rejuvenate, but it would depend on the health of the southern sections.

"There's still a lot of reef here which could supply the propagules for the reef up north to recover, but it's likely to take a very long time because the scale of the event around Lizard Island and further north was so large," he said.

"It will also depend on it not bleaching again, particularly in those areas that are still in good condition."

However, Professor Baird said most of the reefs surveyed off the coast of Townsville and further south were only lightly bleached and now in reasonably good condition.

"That's a positive, but with the current trajectories of carbon dioxide and ocean temperatures, there's nothing to say that those areas might not bleach as soon as next year," he said.

The researchers said the final death toll from the bleaching in the north would not be known until all surveys were completed in mid-November.

They said it was already clear that this event was much more severe than the two previous bleachings in 2002 and 1998.