When Lisa Smiler went fishing in Wattie Creek, she certainly wasn't expecting to catch a 2.7-metre sawfish.

Gurindgi ranger Philip Jimmy said Ms Smiler was frightened.

"She thought it was some other thing than a fish," he said.

Wattie Creek is in the Kalkarindji area, 800 kilometres south of Darwin.

The finding of the one of the world's largest fish so far from the ocean was a surprise to scientists.

Wattie Creek — 800km south of Darwin, 500km from the mouth of the ocean

Biologist Peter Kyne from Charles Darwin University went with ranger Philip Jimmy to the site of the discovery, far up-stream in a Victoria river tributary, out on the edge of the Tanami desert.

"We had no understanding that sawfish occurred so far inland," Mr Kyne said.

"This is the furthest inland that sawfish have been recorded in Australia."

Biologist Peter Kyne from Charles Darwin University went with ranger Philip Jimmy to the site of the discovery, far upstream in a Victoria River tributary, out on the edge of the Tanami desert. ( Supplied: Felicity Meakins )

But Mr Jimmy, who grew up in the area, had seen it before as a teenager, depicted in rock art.

"It happened when I was a boy, walking around the country," he told the ABC.

"I was walking in this creek and I found this painting of this sawfish on the other side of the creek, and I could see it really clear."

The saw fish, known as "kunpulu" in the Gurindji language, has been documented in the area in Indigenous rock art found on Revolver Creek, a tributary of the Victoria River, near Kalkarinji ( Supplied: Felicity Meakins )

Mr Jimmy said a hunter had probably painted the giant fish to remind himself and others of the creature's existence in these waters.

Going, going...

The largetooth sawfish, which is a type of shark, is extinct in all but three of the 80 countries where it was once found.

It is threatened in Australia, but how many are left is not known.

The adults live in the ocean, where their long saws can get caught in fishing nets.

Despite that, the population here is considered more viable than that of its surviving brethren in Papua New Guinea and Brazil, where the fish is targeted by fishermen.

Its depleted population can be found in northern Australia, but Lisa Smiler's family and many other locals had never seen one of the fearsome looking animals, with its external teeth lining the long saw or rostrum of its nose.

"There's probably a generational gap in knowledge," Mr Kyne said.

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Dr Kyne took a DNA sample from Lisa Smiler's sawfish, to help work out the population numbers.

"We basically look at the relationships of individuals and then estimate the population size based upon the number of mothers that have produced the juveniles that we've seen," he explained.

Indigenous language used to track a very big fish

To help biologists learn more about the sawfish population in Northern Territory waterways, a linguist is working with Aboriginal people.

Lisa Smiler shows Dr Peter Kyne and Gurindji rangers Helma Bernard and Nikita Smiler the location of Wattie Creek (a tributary of the Victoria River) where she caught the sawfish in early 2017. ( Supplied: Felicity Meakin )

Dr Felicity Meakin is an ARC research fellow at the University of Queensland, and she's currently in Kalkaringi.

"What I do is a sort of linguist DNA in a sense and that kind of information can be used to corroborate information from Gurindgi people and from scientists."

The traditional Gurindji name for the fish is "kunpulu", the same word used in languages along the Victoria river.

"We can use that linguistic information to track the path of the fish," Dr Meakin said.

The information corroborates the findings of Western science, and is then used to help inform the Federal Government on how best to manage the habitat.