The Gloria Knolls Slide is at least 300,000 years old and 32 cubic km in volume, or 30 times the size of Uluru.
The landslide could also have triggered a large tsunami, said the international team behind the find.
The scientists said debris from the landslide, found as deep as 1,350m below the sea, also provided clues about hidden marine life.
The team made the discovery while conducting three-dimensional mapping of ancient reefs in the Queensland Trough, a vast basin adjoining the Great Barrier Reef Coral fossils.
Dr. Robin Beaman, from Australia’s James Cook University, said the researchers located a cluster of hills, or knolls, more than 1,100m beneath the surface.
According to Beaman, “What we discovered was the smoking gun. It was quite clear that those knolls were the remains of a very large undersea landslide that had occurred some time ago.”
That time was at least 300,000 years ago, he said, because coral fossils collected from the knolls went back that far, and the landslide would have predated them.
Other evidence of the landslide would have been buried over time, he said.
The research, published in the journal Marine Geology, said the landslide had the potential to cause a large tsunami.
“If it was in existence at the time of this landslide, it would have done a similar job.”
He said the future risk to the Queensland coast appeared unlikely because it was “a very old event”, but it was a worthy topic for future research.
The researchers found deep marine life including cold-water corals, mollusks, and barnacles were thriving on the knolls.
He said possibilities for future research were exciting.
The research was a collaboration between James Cook University, University of Sydney, University of Granada, University of Edinburgh and the Australian Nuclear Science and Technology Organisation.