A prehistoric crocodile measuring more than five metres long, dubbed the "swamp king", ruled south eastern Queensland waterways in Australia only a few million years ago, new research has found.

Researchers at University of Queensland identified the new species of prehistoric crocodile -- which they named Paludirex vincenti -- from fossils first unearthed in the 1980s.

"Its fossilised skull measures around 65 centimetres, so we estimate Paludirex vincenti was at least five metres long," said Jorgo Ristevski from University of Queensland's School of Biological Sciences.

"The largest crocodylian today is the Indo-Pacific crocodile, Crocodylus porosus, which grows to about the same size.

"But Paludirex had a broader, more heavy-set skull so it would've resembled an Indo-Pacific crocodile on steroids."

Paludirex was one of the top predators in Australia a few million years ago, capable of preying on giant prehistoric marsupials.

Various species of prehistoric crocodylians had existed in Australia, said study co-author Steve Salisbury from University of Queensland.

"Crocs have been an important component of Australia's fauna for millions of years," Salisbury said.

"But the two species we have today -- Crocodylus porosus and Crocodylus johnstoni -- are only recent arrivals, and were not part of the endemic croc fauna that existed here from about 55 million years ago."

Ristevski said they named the species after Geoff Vincent who discovered the giant fossilised skull near the town of Chinchilla.

"In Latin, 'Paludirex' means 'swamp king', and 'vincenti' honours the late Mr Vincent," he said.

The research was published in the open access journal PeerJ.

"Whether Paludirex vincenti went extinct as a result of competition with species like Crocodylus porosus is hard to say," said Salisbury.

"The alternative is that it went extinct as the climate dried, and the river systems it once inhabited contracted -- we're currently investigating both scenarios."