The diprotodon, an Australian megafauna related to the wombat and koala, is estimated to have roamed Australia as recently as 46,000 years ago. Diprotodons were rhino sized, weighing up to 2.8 tonnes.

Snowy Monaro council biosecurity officer Neil Murdoch was performing a routine weed inspection on Friday in the northern end of the council region when he saw what looked like fossil bones sticking out of the dirt.

It was found on a site that had been previously excavated for megafauna fossils, with three to four of the sites existing cross the region.

"They had just been exposed by erosion over time," Mr Murdoch said.

"It's not something you find every day."

Mr Murdoch came back the next day with an old ecologist friend, Dr Joe Henry, believing they were a creature's lower back before brushing the dirt away to see the diprotodon's unmistakable giant front teeth.

"I think Joe wanted to name it after himself," Mr Murdoch said.

"He was pretty stoked. We were all pretty pleased with the whole thing."

Mr Murdoch then contacted Australian Museum palaeontologist Dr Matthew McCurry who travelled to the region to take a plaster cast of the fossil.

Dr McCurry said the diprotodons were herbivores that resembled giant wombats and were widespread across the mainland, from east to west.

He said they ate a variety of Australian native plants, something researchers had learned by analysing diprotodon pee.

"We don't really know how they went extinct," Dr McCurry said.

"Some people think they might have been hunted to extinction by humans. Other people think they might have gone extinct due to fluctuations in climate."

"They're an example of Australian megafauna. So throughout some of Australia's history, there were these quite large mammals that roamed the landscape."

"It was probably one of the most common megafauna."

Dr McCurry said some people hypothesised they roamed the continent as late as 20,000 years ago.

He said fossilised juveniles helped teach researchers more about how the megafauna grew up, how they changed shape and what stages of their lives they grew.

The 40 centimetre long jaw was now with the Australian Museum.

"It's not the only juvenile, but there are very few that size in the collection," Dr McCurry said.

Dr McCurry said there were better preserved diprotodons but this was still a fossil in good shape. He did not know how old it was.

"It's exciting," he said.

"There's not a lot of juvenile diprotodons that have been found."

Mr Murdoch said he would like to see the discovery site preserved and more excavating done to see if there were more fossils to be found.

The Canberra Times