A distraught motorist phoned triple-0 on Sunday night to say she believed she had just struck a man who was standing in the middle of the Hume Motorway in the southern highlands.
But police could not find any trace of the victim when they arrived about 7pm.
They called in Fire and Rescue NSW, as well as police and rescue helicopters, to search a one-kilometre section of the highway and its shoulder. The search was called off at 10.30pm on Sunday but continued on Monday morning.
Police officers found the body of a kangaroo by the side of the road on Monday morning, and they suspect this was what the woman hit.
However, they have appealed for anyone in the area to check in with family or friends and call police if anyone did not arrive home as expected on Sunday night.
A NSW Police spokeswoman said the woman, 44, was driving north on the Hume Motorway at Yerrinbool near Mittagong, north of the intersection with Sierra Street, about 7pm when she hit what she believed was a man standing in the right-hand lane.
She stopped and phoned triple-0.
Police said the side mirror of her Hyundai hatchback was damaged, suggesting she had struck something.
Police said she was taken to Bowral and District Hospital for mandatory blood and urine testing.
They have urged anyone with information to call Crime Stoppers on 1800 333 000 or visit the Crime Stoppers online reporting page.