The record-breaking heatwave now roasting much of north Queensland has authorities bracing for what would be the third mass coral bleaching on the Great Barrier Reef in four summers.

Cooktown hit 40 degrees on Thursday, the first time the town has notched four consecutive days above 40, adding to the many records broken during the current hot spell that is forecast to run another week.

Extreme heatwave has authorities on edge about a third mass bleaching event in the Great Barrier Reef in four summers. Credit:Dean Miller, GBR Legacy

David Wachenfeld, chief scientist of the Great Barrier Reef Marine Park Authority, said the "amazing temperatures over land" were contributing to water temperatures running a degree above normal in the region.

"It will cause the Great Barrier Reef to get warmer, but how much warmer, how quickly and how long it will last remains to be seen," Dr Wachenfeld said.