The fossilised jaw of a juvenile Diprotodon — or giant wombat — has been discovered at a site known for producing megafauna fossils, in the Monaro region of NSW.

Key points: Council workers have found a fossilised section of a diprotodon's jaw in southern NSW

Council workers have found a fossilised section of a diprotodon's jaw in southern NSW It was found in an area rich with fossils but kept secret from the public

It was found in an area rich with fossils but kept secret from the public The cause of the diprotodon's demise remains open to debate

Two Snowy Monaro Regional Council workers found the fossil on Friday at a place known for its paleontological richness, but kept secret from the public.

The workers alerted the Australian Museum, where hundreds of thousands of fossils collected around the country are kept.

Museum curator Matthew McCurry, who extracted the fossil on Saturday, said the find was of interest because most of the right side of the animal's jaw was intact, and because it was a juvenile animal.

Diprotodons were a species of megafauna that died out thousands of years ago, though the exact time of their extinction is hotly contested, with estimates ranging from 7,000 to more than 40,000 years ago.

Their bones have been found across Australia.

"We find a lot of adults, but also they're mainly fragmentary, so we find a lot of teeth or parts of jaws, but rarely do we find an entire jaw," Dr McCurry said.

"The fossil record is really quite incomplete, so very few animals turn into fossils and when they do, often they break up, and we only find little bits of them.

"Finding new fossils just gives us more of this complete picture about how things evolved — it's just kind of piecing together bits of the story."

The find was made by two council workers in an area known for its fossils. ( Supplied: Australian Museum )

The fossil was unlikely to be kept on public display, and will be made available to researchers at the museum.

Megafauna — including oversized kangaroos and mega-lizards — died out thousands of years ago, but the Diprotodon could have survived on the Liverpool Plains of NSW until about 7,000 years ago, according to the Australian Museum.

The cause of the megafauna's demise remains open to debate.

One theory is that they were hunted to extinction by humans, another that they fell victim to extreme climate fluctuations, or perhaps they succumbed to a combination of the two.

Site a secret well kept by scientists

Dr McCurry said it was not clear why the site was known for its fossil findings, other than having the right soil composition for preservation.

"It looks like a riverine deposit, so they've probably been washed down a river somehow," he said.

"You'd probably need a much larger excavation where you look at the different types of sediment and things like that."

Diprotodons died out several thousand years ago. ( Supplied: Laurie Beirne )

Amid concerns members of the public would attempt to do their own excavations in the area, the fossil site is kept secret by scientists and the council.

Dr McCurry said the council workers who found the fossil did the right thing and reported the finding immediately, without touching the bones.

"They saw something that they thought was important in terms of a geological heritage for the country," he said.

"It means we can collect it safely in the right way and make sure it's available for researchers in the future."