The Great Barrier Reef Marine Park Authority said most of the marine park had been hit by "significant heat stress" over the summer, with water temperatures as much as 2-4 degrees Celsius above average.
The Great Barrier Reef has again been hit with "widespread" bleaching, authorities have said, as higher-than-average ocean temperatures off Australia's northeast threaten the already struggling World Heritage site.
Surveillance flights revealed damage due to heat stress ranging from minor to severe bleaching across the 2,300-kilometre (1,243-mile) network of corals, the Great Barrier Reef Marine Park Authority said on Friday.
"Bleaching has been detected across the Marine Park — it is widespread but variable, across multiple regions, ranging in impact from minor to severe," the authority said in its weekly update.
"The most heavily impacted reefs are around the Townsville region. There have also been reports of early mortality where heat stress has been the greatest."
Over the past week, sea temperatures at the marine park were 0.5-2 degrees Celsius above average. The far north and inshore areas recorded temperatures 2-4 degrees above average.
The news comes ahead of a trip to the reef to inspect the site's health. The recent changes have not yet been classified as a mass bleaching event.
Australia last year dodged an "in danger" listing for the reef for a second time, after heavy lobbying by Canberra led UNESCO to postpone a decision to this year.
The government has announced new funding to prevent the climate-ravaged reef — the world's largest living structure, visible from space — from being removed from the heritage list.
Bleaching occurs when healthy corals become stressed by spikes in ocean temperatures, causing them to expel algae living in their tissues, draining them of their vibrant colours.
There have been five mass bleaching events across the Great Barrier Reef triggered by unusually warm sea temperatures since 1997, leaving many affected corals struggling to survive.
Several cyclones have also battered the reef, as climate change drives more extreme weather. Outbreaks of crown-of-thorns starfish have also eaten away at the coral.
The Australian Marine Conservation Society described the new development as "disastrous news", particularly during a La Nina weather pattern, which is usually associated with cooler ocean temperatures.
"It shows the consistent pressure our reef is now under from global heating," the society's campaign manager Lissa Schindler said.
"A healthy reef can recover from coral bleaching but it needs time. More frequent marine heatwaves primarily driven by the burning of coal and gas means it is not getting this time."