By Agence France-Presse
A swathe of corals bleached in the northern third of the 2,300-kilometre (1,429-mile) long biodiverse site off the Queensland state coast died after an unprecedented bleaching earlier this year as sea temperatures rose.
And researchers who returned to the region to survey the area this month said "many more have died more slowly".
"In March, we measured a lot of heavily bleached branching corals that were still alive, but we didn't see many survivors this week," Andrew Hoey of the Australian Research Council Centre of Excellence for Coral Reef Studies at James Cook University said in a statement.
"On top of that, snails that eat live coral are congregating on the survivors, and the weakened corals are more prone to disease. A lot of the survivors are in poor shape."
Researcher Greg Torda said of the reefs surveyed near Lizard Island off Cairns -- a gateway to the giant ecosystem -- the amount of live coral covering them fell from about 40 percent in March to under five percent.
It is the third time in 18 years that the World Heritage-listed site, which teems with marine life, has experienced mass bleaching after previous events in 1998 and 2002.
The researchers said even though they were still assessing the final death toll from bleaching in the north, "it is already clear that this event was much more severe than the two previous bleachings".
They expect to complete all their surveys by mid-November.
Bleaching occurs when abnormal environmental conditions, such as warmer sea temperatures, cause corals to expel tiny photosynthetic algae, draining them of their colour.
The reef's northern 700-kilometre section bore the brunt of the breaching during March and April, with the southern areas "only lightly bleached and remain in good condition", the scientists added.
"As we expected from the geographic pattern of bleaching, the reefs further south are in much better shape," said Andrew Baird, who led the re-surveys of reefs in the central section.
"There is still close to 40 percent coral cover at most reefs in the central Great Barrier Reef, and the corals that were moderately bleached last summer have nearly all regained their normal colour."
The reef is already under pressure from farming run-off, development, the coral-eating crown-of-thorns starfish as well as the impacts of climate change, with a government report last week painting a bleak picture of the natural wonder.