A traveller discovered the lower section of leg on the Stuart Highway in Coolalinga, 30km south of Darwin, about 9am on Tuesday.

"We've had a forensic pathologist attend on-site who has confirmed the remains are a lower leg of a dark skinned human being," Detective Senior Sergeant Paul Morrissey told reporters.

Police have not found any other body parts in the area despite extensive search efforts and don't know if the victim is dead or alive.

Det Snr Sgt Morrissey said the person was likely to have suffered a serious injury.

"We believe it's within the last 24 hours," he said.

"There have been no reports to hospitals or clinics with an injury of this severity."

A crime scene was declared at the site and the highway linking Darwin to Alice Springs and Adelaide was closed in both directions.

Major crash and crime scene investigators and crime detectives were also called to the scene. General duties and Tactical Response Group officers helped search the area, concentrating on the grass and tree covered nature strip separating the four-lane roadway.

Police have requested motorists with dash cam footage recorded on the section of highway between Monday afternoon and Tuesday morning to come forward.

Australian Associated Press