Corals in the far north of the Great Barrier Reef have started to bleach, leaving the federal government's lead reef agency at a state of "heightened alert" about the prospect of a mass bleaching event unfolding over the next two to four weeks.

David Wachenfeld, chief scientist of the Great Barrier Reef Marine Park Authority, said survey teams had found "significant bleaching" at three reefs in the Shelburne Bay/Wuthathi region of Cape York in the far north.

Rising risk of coral bleaching off Australia and elsewhere in the western Pacific. Credit:National Oceanic and Atmospheric Administration

However, with most of the Great Barrier Reef park now at 2 to 3 degrees above normal, the heat stress on corals that causes bleaching and possible death is rapidly mounting in many regions, Dr Wachenfeld said.

The current forecasts for the next few weeks "are giving us a heightened risk of a mass bleaching event", he said, adding the threat may peak in the second week of March or later.