The Dromornis murrayi could grow to weigh 250 kilograms before it eventually evolved into the world's biggest bird, growing to a staggering 650 kgs.

“It was the first member of the lineage of Dromornis species,” says Dr Trevor Worthy, from Flinders University in South Australia, who led the study.

“Originally, it was the smallest, at a pretty hefty 250 kg, but by eight million years ago it had evolved into D. stirtoni, which averaged a whopping 450 kg – with some individuals reaching 650 kg – the largest birds the world has known.”

Dr Worthy’s team of palaeontologists, from Finders University and University of New South Wales, revealed the news in a study just published in the Journal of Vertebrate Paleontology.

“Mihirungs were giant flightless birds only found in Australia and are known only from fossils,” said Dr Worthy. “The largest stood two metres high at its back and reached well over three metres at the head.

“The last species died out, probably about 50,000 years ago.”

Seven species in four genera of the bird are generally recognised, with the last five named in 1979 by Patricia Vickers Rich, now a Professor at Monash University.

The new species Dromornis murrayi is named after Peter Murray, former Northern Territory Museum curator and co-author of the book Magnificent Mihirungs, bringing the total number of mihirungs known to eight.

“This is one monstrous chook that would have had no trouble kicking anyone’s dunny door down,” said Professor Mike Archer from the University of New South Wales.

Professor Archer said Riversleigh was an ideal environment to find fossils.

“We're discovering more and more, as I say, the preservation at Riversleigh, something weird was happening, such that we find even cells in these things, fossilised cells with the organelles like nuclei in them,” he told the ABC’s PM program.

Professor Suzanne Hand, also from University of New South Wales said the very large and distinctive bones of Dromornis murrayi are quite common in the Riversleigh fossil deposits, and are easily spotted by scientists and visitors to the site.