Mass bleaching has destroyed as much as 35% of the coral on the northern and central Great Barrier Reef, scientists believe.
Just 7% of the entire Reef avoided any damage as a result of bleaching, Australian experts said on Monday.
This latest occurrence is the third time in under 20 years that the Great Barrier Reef has experienced mass bleaching "due to global warming".
Bleaching occurs when the water is too warm, forcing coral to expel living algae and causing it to calcify and turn white.
Mildly bleached coral can recover if the temperature drops, otherwise it may die.
The Great Barrier Reef, a designated World Heritage Site, attracts about £2.5 million in tourism a year.
Following extensive aerial surveys and dives to access damage across 84 reefs in, scientists said the impact of bleaching is more severe than they expected.
Professor Terry Hughes, director of the ARC Centre of Excellence for Coral Reef Studies at James Cook University, said the current trend is "much more extreme" than anticipated.
Although the impact has been exacerbated by one of the strongest El Nino weather systems in nearly 20 years, which recently subsided, scientists believe climate change is the underlying cause.
UNESCO's World Heritage Committee last May stopped short of placing the Great Barrier Reef on an "in danger" list, but the ruling raised concern about its future.