The National Coral Bleaching Taskforce says 95% of reefs from Cairns to Papua New Guinea are now severely bleached.
It says only four reefs out of 520 have no evidence of bleaching.
Unesco voted not to put the reef on its World Heritage in Danger list last year, but green groups want the decision reassessed.
Coral taskforce convener Professor Terry Hughes told the BBC his team was yet to find the southern border where the bleaching ended.
"Tomorrow we will continue further south from Cairns to Townsville about 400km (250 miles) and spend the day scoring another 150 reefs," Professor Hughes said.
Experts say it is too early to tell whether the corals will recover, but scientists "in the water" are already reporting up to 50% mortality of bleached corals.
Climate change and the effects of El Nino are being blamed for the rise in sea temperatures that causes coral bleaching.
(BBC News)
