NSW biologists have identified a previously unknown species of platypus – the fifth and biggest so far – from a tooth found at a renowned fossil site in Queensland’s gulf country.

“It would have been one of the biggest animals by far in those ancient waterways,” said University of NSW palaeontologist Mike Archer.

“Only crocodiles would have been bigger. Everything else would have thought twice about going for a swim with this platypus-zilla.”

Before the latest find, three extinct platypus species were known from fossils discovered in Central Australia’s Simpson Desert, Argentina’s far south and the Riversleigh World Heritage Area in Queensland. The new species is thought to have lived between five and 15 million years ago, reaching a metre in length.