'Gut-wrenching' bleaching hits Great Barrier Reef again

Almost three-quarters of the Great Barrier Reef has been hit by coral bleaching, a report says. (HANDOUT/WWF AUSTRALIA)

Scientists fear time is running out to protect the Great Barrier Reef, which is enduring one of its most extensive coral bleaching events.

The full extent of the impact is still unknown despite a "gutwrenching" update on Wednesday which revealed almost three-quarters of the reef has been affected.