The Gloria Knolls Slide is at least 300,000 years old and 32 cubic km in volume, the BBC reports.
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 (4,430ft) 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.
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.
“What we discovered was the smoking gun,” he told the BBC.
“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.
He described it as “catastrophic collapse” because the knolls – as long as 3.6km (2.2 miles) – were found 30km from their original location.
Other evidence of the landslide would have been buried over time, he said.
Was there a tsunami?
The research, published in the journal Marine Geology, said the landslide had the potential to cause a large tsunami.
Although modelling had put its elevation as high as 27m, the impact of a tsunami would have been significantly offset by the presence of coral reefs.
“The Great Barrier Reef acts like a giant porous breakwater to reduce the energy [of ocean swell],” said Dr Beaman. “If it was in existence at the time of this landslide, it would have done a similar job.”
He said future risk to the Queensland coast appeared unlikely because it was a “a very old event”, but it was a worthy topic for future research.
Deep marine life
The researchers found deep marine life including cold-water corals, molluscs and barnacles were thriving on the knolls.
The corals, unlike their shallow reef counterparts, could survive in 4C temperatures with no sunlight, Dr Beaman said.
He said possibilities for future research were exciting.
“That really is the next frontier,” he said.
“We probably have a bit of and idea of what’s living up to 200m or 300m [deep], but beyond that, very few people have done much work in this area.”
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.