The Great Barrier Reef is in the worst state it's been in since records began and will be "pretty ugly" within 40 years, Australian scientists say.

A senate committee is investigating how the Australian and Queensland governments have managed the reef, with the UNESCO agency to decide next year whether to list it as a World Heritage site in danger.

Great Barrier Reef. Credit:Tourism and Events Queensland

Scientists have told the committee the reef is facing threats from coastal development, such as a massive port-related dredging project at Abbot Point, farm run-off and poor water quality.

The reef cannot rejuvenate after times of stress as it once did, the scientists say.