Last year, the same thing killed off 22 percent of its corals.
"This is the first time we have ever seen bleaching in back-to-back summers", said Dr Neal Cantin, from The Australian Institute of Marine Science.
When sea surface temperatures rise, corals expel the colourful algae. Also, there is no way to know how many could recover as algae have been overgrowing on numerous corals already.
"I've been photographing this area of the reef for several years now and what we're seeing is unprecedented", Brett Monroe Garner, a marine biologist working with Greenpeace, said.
It follows pictures of newly bleached coral taken in recent weeks at Moore Reef, near Cairns, by the reef scientist Tyrone Ridgway, as well as by divers further south near Palm Island.
Australian coral reef experts made that conclusion after flying over the Great Barrier Reef for six hours yesterday, according to a press release issued by the Great Barrier Reef Marine Park Agency (GBRMPA).
Scientists in Australia have uncovered widespread damage to coral in the Great Barrier Reef, warning that "we are entering uncharted territory", after surveys showed consecutive years of bleaching for the first time. Water temperatures have now been above average for at least 12 months in north Queensland waters.
"We can't afford to sit by and watch climate change drive all the world's coral reefs to extinctions by the end of the century", Julia Baum, a reef researcher at the University of Victoria, told Climate Central a year ago.
"Just a few months ago, these corals were full of colour and life".
Bleaching happens when warm waters cause the coral to oust algae living in their tissues.
An aerial survey showed severe bleaching in the middle of the reef, although it may not have had time to recover fully from previous bleaching. Now, everywhere you look is white.
The bleaching is part of a global event affecting the world's coral reefs over the past two years.
"We have on our doorstep the clearest signal that climate change is happening, and that governments aren't moving fast enough to stop it", Elst said.
"We are working closely with the Commonwealth to ensure we gather all the information required to determine the extent of this latest bleaching event", Mr Miles said.
World Wildlife Fund Australia called on the federal government to accelerate solar and wind energy use by setting a 100 per cent renewable energy target by 2035.
Environmental groups are acting against the proposed construction of the largest coal mine in Australia, which could significantly contribute to the greenhouse gas emissions.