Coral bleaching driven by climate change has hit three quarters of the Great Barrier Reef, with 50 per cent suffering high or very high levels of damage.
The reef is in the grip of its fifth mass bleaching event in eight years due to heat stress fuelled by the burning of fossil fuels.
Evidence of the latest event has been emerging over the past couple of months but the latest overview from the Great Barrier Reef Marine Park Authority shows just how bad it is.
The update comes amid ongoing efforts by the federal government to keep the reef of the list of World Heritage sites in danger.
In-danger listings are not supposed to be a punishment, rather an instrument to encourage a redoubling of conservation efforts.
Aerial surveys of hundreds of reefs in the Great Barrier Reef Marine Park show half have high or very high levels of coral bleaching.
The authority said less than 10 per cent had extreme levels of bleaching but it did not provide a precise figure.
Only a quarter of surveyed reefs showed no to low levels of bleaching.
In-water surveys suggest the same thing with most detecting moderate to severe impacts and the danger is not over yet with sea surface temperatures remaining up to 1.5C above average for this time of year.
"A build-up of heat stress is again starting to accumulate in the northern and offshore central region but continues to plateau in the southern region," the authority said.
The damage from climate change is being compounded by outbreaks of coral-munching crown-of-thorns starfish.
The authority also warns the reef could suffer flood-related harm after above average March rainfall in Queensland's far north.
The Australian Marine Conservation Society says the results are alarming but not surprising given sea temperatures were up to 2.5C above the long-term average across the marine park this summer.
“This is probably the worst we have seen since 2016," said the society's reef campaigner Dr Lissa Schindler.
"We need urgent climate action and the reef’s custodian, the Great Barrier Reef Marine Park Authority, should be sounding the alarm for Australia to do more."
Meanwhile, the Greens are demanding details about a very recent trip - by the government's special envoy for the reef Nita Green and the marine park authority's CEO Josh Thomas - to UNESCO in Paris.
"The Greens are calling for full transparency of any recent lobbying of UNESCO by the Albanese government in relation to the reef," Senator Peter Whish-Wilson said.
"Successive governments have gone to extraordinary lengths to stop an in-danger listing of the reef by deliberately deceiving the world of the severity of climate change impacts on the reef, while at the same time approving massive new fossil fuel projects."
Environment Minister Tanya Plibersek's office did not directly answer AAP's questions about the purpose of the visit.
In February, Australia sent an update on its reef protection measures to UNESCO's World Heritage Centre and the International Union for Conservation of Nature.
The organisations will assess those efforts and make recommendations to the World Heritage Committee when it revisits the in-danger listing in July.
In a statement on Saturday, Ms Plibersek said climate change was the biggest threat to the world's coral reefs including the Great Barrier Reef.
She said the government was acting by committing to net zero and investing in renewable energy and efforts to boost reef health, adaptation and resilience.
