The Great Barrier Reef is in the midst of its worst coral bleaching event on record, but at least one tourist body says scientists' claims are overstated.

Some 1300 kilometres of reefs from Papua New Guinea south to Townsville have been surveyed by air, with scientists painting a bleak picture.

Monitoring the effect of coral bleaching is crucial to determining the best plan of action. Credit:XL Catlin Seaview Survey

" ... Seventy-five per cent of corals north of Cairns are snow white," Professor Terry Hughes, convenor of the National Coral Bleaching Taskforce, said on Monday.

"The amount of corals in the northern third (of the reef) used to be substantially higher than further south. That's no longer the case."