A security camera has captured footage of a boy in nothing but a nappy sitting on the roof of a car driven by his mother, Nine News reported.
The incident reportedly happened at about 5.45 pm on 4 December on a Perth highway between the suburbs of Camillo and Harrisdale.
According to witnesses, the baby was totally "enjoying himself" while clinging to the roof rack and "oblivious to the danger he was in".
"Police observed three children in the car, two secured in child restraint seats however it will be alleged that a four-year-old boy was unrestrained. Other motorists managed to get the attention of the driver who stopped and placed the child in the car before driving away", the police said in a report.
It's believed that the 36-year-old mother was unaware that the child was on top of the moving car.