The Great Barrier Reef is facing a critical period of thermal stress with hundreds of sightings of coral bleaching reported in February.

Significant bleaching has been discovered on three reefs in the far northern portion of the world's largest coral reef ecosystem.

The next four weeks will be critical as temperatures in the Coral Sea remain significantly above average, says Great Barrier Reef Marine Park Authority chief scientist Dr David Wachenfeld.

"Our reports of coral bleaching from the field continue to show the same pattern as well reflecting the pattern of heat stress," he said.

He said there had been 250 reports about bleaching in the last month through the organisation's "Eye on the Reef" monitoring program.

Leon Zann

The outlook for March is "concerning" with sea temperatures in parts of the marine park two to three degrees above the March average.

Forecasts from the weather bureau indicate ongoing levels of thermal stress for at least the next two weeks and maybe even longer, Dr Wachenfeld said.

"This still is a critical time for the reef and it is the weather conditions in the next two to four weeks that will determine the final outcome," he said.