Known as megafauna, the huge animals were driven into extinction by a steady warming of Australia's climate, which in turn saw a once-lush outback region turn to red desert and grasslands.

"For about the last half-million years it's been consistently getting drier in Australia," Dr Gregory Webb told Reuters after studying fossil-rich areas of south-east Queensland state.

"The apparent progressive megafaunal extinction on the Darling Downs does not support the sudden blitzkrieg model resulting from human hunting," Webb's report said.

The megafauna -- kangaroos 2.5 metres (8 feet) tall, wombats as big as cars and cattle, giant Ostrich-like Emus and lizards -- were common in vast areas of Australia 40,000 years ago before gradually disappearing.

Most theories on their vanishing centre on the arrival in Australia around the same time of Aboriginal people, who were believed to have hunted the animals out of existance.

But Webb, from Queensland University of Technology, said a study done with colleague Dr Gilbert Price had found many animals were probably drought-stressed when they died.

If humans had been responsible, he said, the fossil evidence would show the vulnerable and easily-hunted animals dying out around the same period rather than over thousands of years.

"Whole habitats changed, from forests which required a lot of rainfall to grasslands, and now it has become much more open and scrubbier," Webb said.

"Of course the organisms that required more enclosed lush, green habitat simply had nowhere to live."