The worst Great Barrier Reef coral bleaching event on record extends further south than first thought, a marine expert says.
Professor Terry Hughes, convenor of the National Coral Bleaching Taskforce, is doing an aerial survey of the reef and says the most recent images show damage up to Townsville.
"There's no way to sugarcoat this ... the pictures don't lie," he told AAP.
"We were hoping to find a southern boundary somewhere between Townsville and Cairns. We now know that boundary is further south."
Each of the 74 reefs surveyed between the two cities was on average bleached by 25-30 per cent.
On Tuesday, Prof Hughes described the most pristine parts of the reef, those at the north, as being "fried".
Hughes' team has surveyed nearly 600 reefs between Papua New Guinea and Townsville, and only four have escaped the destructive effects of bleaching.
The damage in the northern part of the Great Barrier Reef far surpasses the previous worst bleaching event, in 2002, when 18 per cent of reefs were ranked in the two most severe categories.
Prof Hughes says the survey is unequivocal proof the warming waters due to climate change are responsible for potentially irreversible damage.
"The northern Barrier Reef is where overfishing and runoff aren't issues - (but) despite being most pristine part, it is suffering from global warming," Prof Hughes said.
"We've been warning the government that global warming is impacting on reefs. This is the third time it has happened."
He says more mass bleaching events are likely to occur in the next 10 years, more frequently than the time it takes the reef to recover.
The aerial survey will continue south from Townsville in coming days.
"We know that the southern tip is not bleached. (The) only way to get a full-scale picture is to do an aerial survey like this," Prof Hughes said.
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