Whanganui man Ammar Saleh noticed a car parked in the breakdown lane on his way to New Plymouth on Saturday - and then on fire when he returned.
"When I drove back at around 4.30pm it was on fire," he said.
"It looked like it hadn't been burning long and I wondered how it caught on fire."
Saleh said no one was in or near the burning car.
Whanganui police and firefighters arrived at 4.45pm.