Firefighters rushed to extinguish the intense blaze, which had spread from the car, parked about 15 metres from the side of the highway, into nearby paddocks.
After the fire was put out, the bodies of a woman and two children were found in the hatchback.
A truck driver stopped to help at the scene during the fire, while multiple witnesses called Triple Zero.
The exact circumstances leading up to the fire have not been determined, but it is believed the car was parked when the fire began.
Detectives from the Arson and Explosives Squad are now investigating.
Police said they are “not looking for anybody else at this time”.
Detectives are now calling for any witnesses or anybody who may have dashcam footage of the fire.