Aerial surveys over two-thirds of the reef have confirmed "widespread" bleaching, the Great Barrier Reef Marine Park Authority said on Friday.

Climate change is the biggest threat to tropical reefs worldwide, and coral bleaching is caused by heat stress.

It's not always fatal but corals are likely to die if temperatures remain higher than normal for too long.

Reef Authority chief scientist Roger Beeden said bleaching had been recorded at 300 sites from Cape Melville north of Cooktown to just north of Bundaberg.

"The results are consistent with what we have seen with above average sea surface temperatures across the marine park for an extended period of time," Dr Beeden said.

He said there was prevalent shallow-water coral bleaching on most surveyed reefs, but noted heat stress this summer had varied with location.

But the bleaching mirrors what has happened to other reefs around the world in the past year.

The Australian Institute of Marine Science, which collaborated on the aerial work, said teams needed to get into the water to determine the severity of the bleaching.

The Australian Marine Conservation Society said the latest mass bleaching showed the federal government must lift emissions targets and end fossil fuel projects to keep global temperatures in a survivable range for reefs.

"We need to do much more to address climate change, which is driving the marine heatwaves that lead to coral bleaching," reef campaigner and marine ecologist Dr Lissa Schindler said.

"The former coalition federal government failed to heed the alarm sounded by four mass coral bleaching events.

"Australia's current target of a 43 per cent cut in carbon pollution by 2030 is consistent with a 2C warming pathway, which equates to the loss of 99 per cent of the world's coral reefs."

Australian Associated Press