We knew this news was coming, perhaps. Now that it is here, it is no less shocking.

Ever since a historic coral bleaching event hit the treasured Great Barrier Reef in March - courtesy of a dramatic influx of warm water in the region - scientists have been trying to take a toll of the damage. And the latest report, from researchers with the ARC Centre of Excellence for Coral Reef Studies at James Cook University in Queensland, seems to reaffirm some of the worst fears.

It's important to caution that not all of the evidence is in yet. The Great Barrier Reef is enormous and takes time to survey.

Still it appears that in the northern part of the Great Barrier Reef, large volumes of corals may have died. That's the part of the reef researchers say was, previously, the most "pristine" - in other words, the least damaged by pollution and other human influences.

"In the area [where] I am, I'm at Lizard Island, about 250 kilometres north of Cannes, around about 80 per cent and upwards of the corals have died," said Andrew Hoey, a senior research fellow with the Centre, during a break yesterday from the ongoing research.

In a press release from the ARC Centre, one of Hoey's colleagues, Greg Torda, said "millions" of corals in the northern sector of the reef have died.

Even though their studies are not complete, the researchers are already asserting that this is far worse than prior bleaching events that occurred in 1998 and 2002.

"The mortality is devastating really," said Hoey. "It's a lot higher than we had hoped."