AN ENORMOUS slab of sea floor is in the early stages of collapse off north Queensland, and could generate a tsunami when it finally breaks off, researchers warn.

The one cubic kilometre slab - dubbed the Noggin Block - is the remains of an ancient underwater landslide, and sits perched on the edge of the continental shelf.

Researchers discovered it while mapping the sea floor around the Great Barrier Reef.

James Cook University marine biologist Robin Beaman says the slab will eventually break away from the Great Barrier Reef and when that happens it could generate a huge tsunami.

"It's actually up on the top of the continental slope in about 350 metres of water,'' Dr Beaman told ABC radio.

"It's a pretty big chunk of sea floor (and) is in the very slow, early stages of starting to break away from the edge of the Great Barrier Reef.

"If it were to break away catastrophically, that is break away really quickly ... 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.''

It's estimated it would take about an hour for any tsunami to hit coastal areas such as Mourilyan Harbour and Clump Point, south of Cairns.

But Dr Beaman says it's still very stable, and something like a very large earthquake near the site would be needed to trigger a catastrophic collapse any time soon.

"That is very unlikely. But we should be aware that these things exist. We don't really know when such a block might collapse. All I can say is sometime it eventually will.''