Two-thirds of the Great Barrier Reef has now been devastated by severe coral bleaching, with the most intense damage occurring further south this year, Queensland scientists say.

Key points: Of the 2,300km of reef, 1,500km is bleached

Of the 2,300km of reef, 1,500km is bleached Middle third section has most intense bleaching, southern third unscathed

Middle third section has most intense bleaching, southern third unscathed Caused from record-breaking temperature

Caused from record-breaking temperature The fastest growing coral will take a decade to recover

An aerial survey of the reef's 2,300-kilometre length by the Australian Research Council's Centre of Excellence for Coral Reef Studies showed 1,500km had been bleached.

Compared to its 2016 survey, where the northern third was worst off, this year the central third — Cooktown down to Townsville — recorded the most intense bleaching.

The southern third is unscathed.

The council's director, Terry Hughes, who undertook the aerial survey both years, said record-breaking temperatures, driven by global warming, has led to the bleaching.

He said it would take the fastest growing coral in the 1,500km stretch about a decade to recover.

The main areas of coral bleaching in 2016 and 2017. ( Supplied: ARC Centre for Excellence for Coral Reef Studies )

"The 10-year recovery hasn't even started yet, because of the back-to-back bleaching," he said.

"There has been no gap in bleaching between 2016 and 2017, we need gaps between bleaching to get a full recover."

Dr James Kerry from James Cook University in north Queensland said it was the fourth time the Great Barrier Reef had bleached severely.

The survey estimated that a 1 degree Celsius rise in temperatures had caused four bleaching events in the past 19 years.

"It is fair to say that this is extremely concerning for the long-term health of the reef," he said.

"It is facing a number of stresses, but I think the outside stress is coral bleaching, and we have now seen two major events in two years, which we have never seen before - that is unprecedented.

"In 1998, 2002, 2016, and now in 2017 - bleached corals are not necessarily dead corals, but in the severe central region we anticipate high levels of coral loss."

Coral bleaching off Mission Beach, about two hours south of Cairns. ( Supplied: Bette Willis )

Coupled with the 2017 mass bleaching event, Tropical Cyclone Debbie struck a section of the Great Barrier Reef in March.

Scientists said the intense, slow-moving system was likely to have caused varying levels of damage along a path up to 100 kilometres in width.

Researchers said any cooling effects related to the cyclone were likely to be negligible in relation to the damage it caused, which unfortunately struck a section of the reef that had largely escaped the worst of the bleaching.

Sorry, this video has expired Footage shows areas of the Great Barrier Reef in the Whitsundays devastated by Cyclone Debbie ( Supplied: GRMPA )

Search for unspoiled reefs

Mark Read, the Great Barrier Reef Marine Park Authority's manager of species conservation, said it the authority was working with tourism operators to find reefs that had not been affected by bleaching.

"To see whether or not there are areas we need to put additional resources towards protecting," he said.

"Sure there's some areas that have been hit incredibly hard by the bleaching and the cyclone, but it's still a magnificently large place with some areas that have been largely unaffected and that are still in very good condition."