Scientists have unearthed Australia's oldest known crocodile eggshells which may have belonged to drop crocs - creatures that climbed trees to hunt prey below.

The discovery of the 55-million-year-old eggshells was made in a sheep farmer's backyard in Queensland with the findings published in the Journal of Vertebrate Paleontology.

The eggshells belonged to a long-extinct group of crocodiles known as mekosuchines, who lived in inland waters when Australia was part of Antarctica and South America.

Co-author Prof Michael Archer said drop crocs were a bizarre idea but some were perhaps hunting like leopards - dropping out of trees on any unsuspecting thing they fancied for dinner.

Prof Archer, a palaeontologist at the University of New South Wales, said mekosuchine crocodiles - which could grow to about five metres - were plentiful 55 million years ago, long before their modern saltwater and freshwater cousins arrived in Australia approximately 3.8 million years ago.

The drop croc eggshells were discovered several decades ago but only recently analyzed with the support of scientists in Spain.

It's a bizarre idea, Prof Archer said of the drop crocs, but some were probably terrestrial hunters in the forests.

The findings complement prior discoveries of younger mekosuchine fossils found in 25-million-year-old deposits in another area of Queensland.

Since the early 1980s, Prof Archer has been involved in excavating a clay pit in Murgon, a small regional town approximately 270km north-west of Brisbane, which has become known as one of Australia's oldest fossil sites.

This forest was home to the world's oldest-known songbirds, Australia’s earliest frogs and snakes, small mammals with South American connections, and one of the world’s oldest known bats, Dr Michael Stein, a co-author, noted.

Prof Archer remembers the day in 1983 when he and a colleague stopped by a sheep farm, explaining how they sought to uncover the prehistoric treasures buried beneath the paddock. They received a welcoming response indicating there were more remarkable finds ahead.

With more digging there will be a lot more surprises to come, he concluded.