Aerial and underwater surveys of the Great Barrier Reef have revealed 93 per cent of it has been bleached to some extent.

Aerial checks of more than 900 individual reefs showed the spread varies dramatically along its 2,300 kilometres, from 90 per cent north of Port Douglas to less than 10 per cent south of Mackay.

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

Professor Terry Hughes from the National Coral Bleaching Taskforce said the most severe bleaching had hit the northern section of the reef, which stretches 1,000 kilometres north of Port Douglas.

"We've never seen bleaching on the Great Barrier Reef of that severity and when bleaching is that strong it affects virtually all coral species," Professor Hughes said.

"We expect the central and southern corals to regain their colour and recover over the next few months.

"The southern third of the Great Barrier Reef fortunately cooled down late in summer due to ex-cyclone Winston.

"The 2016 footprint could have been much worse."

But Professor Hughes said the overall picture was much worse than the past two bleaching events.

"In 1998 and 2002, 40 per cent of the reefs had no bleaching — it's only 7 per cent this time," he said.

"We also know in 1998 and 2002 about 18 per cent of the reefs were severely bleaching — this time it's over half.

"So by those metrics this bleaching event is three or four times more severe."

Professor Andrew Baird from the ARC Centre of Excellence for Coral Reef Studies has spent the past 17 days at sea recording the bleaching event.

"Tragically, the northern section is the most remote part of the reef, and its remoteness has protected it from most human pressures but not climate change," he said.

"North of Port Douglas, we're already measuring an average of close to 50 per cent mortality of bleached corals."

Meanwhile, Australia's southern waters are experiencing an extended warm period that scientists say offers a glimpse into Tasmania's climate future.

Water's off the state's east coast have been up to four degrees warmer than average for more than 100 days.