They revealed a mixed picture, with some severe bleaching on reefs closer to shore, but outer "ribbon reefs" in the far north escaping damage.
When subjected to environmental stress, many coral reefs expel their zooxanthellae en masse, and coral polyps are left without pigmentation, appearing nearly transparent on the white skeleton of the animal, a phenomenon known as bleaching.
"The reef had only just begun recovering from impacts in 2016 and 2017 and now we have a third event", the marine park's chief scientist, David Wachenfeld, told the BBC.
Hughes said that extensive bleaching at levels "comparable to 2017" had been found in surveys this week in the central parts of the reef, when it is estimated that around 22 percent of shallow water coral across the 2300 square kilometers of the reef had died.
A new mass bleaching event has occurred at Australia's Great Barrier Reef, the third since 2016, officials said on Thursday. Marine heatwaves are the main cause, but other threats such as farming pollution, coastal development and illegal fishing were also cited.
The Australian Research Council Centre of Excellence for Coral Reef Studies is conducting aerial surveys, and its director Terry Hughes has been posting updates throughout the week.
"We could see that some of those corals were big enough that they must have survived the 2017 bleaching and now they re-bleached", he said.
"The inshore section between Cairns and Townsville is certainly severely bleached".
"Climate change remains the single greatest challenge to the Reef", it said on its website, calling efforts to reduce global emissions "essential" to saving the reef.
Australia, whose government backs coal-fired power and has been criticized for its indifference to climate change, has experienced one of its worst droughts in the last few months and one of the most devastating bushfire seasons in history as well as one of the worst storms in decades, in addition to the ongoing COVID-19 pandemic. In august, the Great Barrier reef's outlook for the first time described as "very poor".
The world's largest reef system has suffered another mass bleaching event.