SYDNEY, Australia — Scientists surveying the Great Barrier Reef said Tuesday that it had suffered the worst coral die-off ever recorded after being bathed this year in warm waters that bleached and then weakened the coral.

About two-thirds of the shallow-water coral on the reef’s previously pristine, 430-mile northern stretch is dead, the scientists said. Only a cyclone that reduced water temperatures by up to three degrees Celsius in the south saved the lower reaches of the 1,400-mile reef from damage, they added.

On some atolls in the north, all the coral has died, said Prof. Terry Hughes, the director of the ARC Center of Excellence for Coral Reef Studies at James Cook University in Townsville, in the eastern state of Queensland. Professor Hughes and a team of scientists drew their findings from about 900 dive surveys along the length of the reef in October and November.

“The good news is that in the south, only about 1 percent of the reef’s coral has died, and the mortality rate in the middle is about 6 percent,” Professor Hughes said. Vibrant color has returned to that coral, and the reef there is in good condition, he added.