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.