Geoscience Australia said the reefs, estimated to be at least 100,000 years old, were a major discovery.

They were found by a survey team that went to the Gulf of Carpentaria to follow up on initial exploration work carried out two years ago, when three "patch" reefs, one 10 kilomores (6.2 miles) across, were found.

"The exciting part of all this is that it really highlights how little we know about the continental shelf around Australia," voyage leader Peter Harris told AFP.

"The water is turbid and deep, in a lot of places we can't see the sea floor -- there's undoubtedly more large areas of these kind of reefs in the waters of tropical Australia that we haven't found yet."

Harris said the reefs were previously unknown because they were about 20 metres (66 foot) under water, making them invisible on satellite photographs.

He said the reef's existence was confirmed by state-of-the-art sonar mapping carried out by Geoscience Australia's ocean research vessel "Southern Surveyor".

"This discovery makes the Gulf of Carpentaria an important modern coral reef region of Australia, encompassing as many as 50 small coral patch reefs, one to 10 kilometres in diameter, plus an elongated platform coral reef that is around 100 kilometres in length extending westwards from Mornington Island," Harris said.

"The thickness and wide distribution of the reefs point to a long history of reef growth extending possibly over the past 100,000 years or more," he added.

Australia is already home to the world's largest coral reef, the Great Barrier Reef, which stretches over more than 345,000 square kilometers (133,000 square miles) off Queensland's coast and is home to 1,500 fish species.

It is considered the world's largest living organism and has been listed by the United Nations as a world heritage site.

However, coral reefs worldwide have been under threat in recent years from coral bleaching, believed to be caused by rising sea temperatures that result from global warming.

Harris said the newly-discovered reefs could be included in a maritime national park being planned for the area.

"We need to know where these reefs are so we can look after them and make sure they're properly managed," he said.

"Coral reefs are among the most biodiverse ecosystems on earth and they need to be protected."

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