The researchers, whose findings were published in the journal Nature Hazards, said the one cubic kilometre slab in the Coral Sea – dubbed the Noggin Block – "eventually will collapse".
The ominous slab was discovered by geologists who have been using 3-D mapping techniques to build a picture of the sea floor along the deepest parts of the reef.
"It is a pretty big chunk of sea-floor [in] the very slow, early stages of starting to break away from the edge of the Great Barrier Reef," said Dr Robin Beaman, a marine geologist at James Cook University.
"If it were to break away catastrophically, that is break away really quickly, what that would do is it would create a surface wave above it. It would actually cause a tsunami. That tsunami would travel across the Great Barrier Reef, it's about 70 kilometres offshore, and it would impact the local area, the North Queensland area."
The Noggin Block, perched on the edge of the continental shelf, is the remnant of an ancient underwater landslide.
Dr Beaman said the slab appeared stable for now and was only likely to move in the near future if there were a trigger such as a very large earthquake nearby. While such an earthquake is "unlikely", it is possible – and the slab will in any case eventually collapse.
"We don't really know when such a block might collapse," he told ABC Radio.
"All I can say is sometime it eventually will." The finding marks the first discovery of an undersea landslide on the Great Barrier Reef.
"We found this one large block that stood out," Dr Beaman said. "It is sitting on top of a submarine canyon, cutting into the slopes and it is in the preliminary stage of collapse We're not trying to alarm people, but we need to know it is there and what could happen when it falls."