The Great Barrier Reef is dying on our watch, and that's almost all we're doing.

The scale of the second mass bleaching in two years won't be known for several weeks but early indications suggest the corals in the tourist heartland between Townsville and Port Douglas have been hammered.

Even as scientists from James Cook University and the reef's Marine Park Authority complete aerial surveys of the 2300-kilometre natural treasure, researchers such as Line Bay from the Australian Institute of Marine Science (AIMS) are returning from dives with grim tidings.

"We saw bleaching on every reef we went to," Bay tells me. "And it was quite significant", reaching as much as 90 per cent in shallow reefs.