Great Barrier Reef is home to 400 types of coral. (File)

Australia's Great Barrier Reef is in very poor condition because of climate change, over fishing and land clearing, a state agency said on Friday, as it downgraded the reef's status to the lowest level, which could jeopardise its World Heritage status.

The Great Barrier Reef Marine Park Authority (GBRMPA) said the health of the world's largest coral reef system, off the northeast coast of the Queensland state, had deteriorated since its last review, in 2014, but the problems the reef faces were not insurmountable.

"This report draws attention to the fact that the outlook for the Great Barrier Reef, the long term outlook, is very poor- that's largely driven by climate change," GBRMPA's Chief Scientists David Wachenfeld told reporters in Sydney.

"Despite that, with the right mix of local actions to improve the resilience of the system and global actions to tackle climate change in the strongest and fastest way possible, we can turn that around."

The report, which is compiled every five years, painted a deteriorating picture of widespread coral bleaching, habitat loss and degradation caused by human-induced climate change, overfishing, poor water quality, and coastal land clearing for grazing.

The reef stretching for more than 2,300 km (1430 miles) is home to 400 types of coral, 1,500 species of fish and 4,000 types of molluscs.

Some parts of the reefs remained in good condition but many species including dolphins, dugongs, sharks, rays and turtles were being threatened.

UNESCO's World Heritage Committee last year called for global action on climate change to protect five large coral reefs, including the Great Barrier Reef.

The committee is due to consider the reef's heritage listing, considering its health and a possible "in danger" status.

"The Great Barrier Reef is one of the globe's most famous World Heritage Areas yet the report finds that its integrity is challenged and deteriorating," environmentalist group Australian Marine Conservation Society said in a statement.

"This is now the third Outlook Report. We've had ten years of warnings, ten years of rising greenhouse emissions and ten years watching the Reef heading for a catastrophe," said the group's director of strategy, Imogen Zethoven.

"This report will be a major input into UNESCO's committee and here is a very strong case for the reef to be considered for the in danger list."

The inclusion of the reef on the in danger list would be an embarrassment for the government and could damage the tourist industry.