Aerial surveys showed that "some southern areas of the Reef that had little or no bleaching in 2016 and 2017 have now experienced moderate or severe bleaching," the authority said.
Warmer temperatures, particularly in February, led to the latest bleaching, the GBRMPA said. Bleaching does not automatically mean the corals will die. Mildly or moderately affected reefs usually recover.
Saving the Great Barrier Reef
Key tourism reefs in northern and central areas should survive, the authorities said.
Ocean warming in 2016 and 2017 killed almost half the coral.