Kerry Quartermaine, a production manager at Action Engineering, was heading out of the city about 10am when he noticed a car ahead of him pulling over.
As the car slowed, he noticed a "big ball of flame" coming from underneath the vehicle.
Mr Quartermaine pulled over as well, rushed up to her vehicle and helped the woman from the car as the flames took hold.
"I saw in her back window she had one of those family stickers, with mum and dad and all the kids, which alarmed me, but she was by herself.
"She was shaken. She didn't even realise the car was on fire. She just heard something go `bang' and the car stopped.
"But it actually blew a hole in the bottom of the motor and all the oil was running out and was on fire."
Mr Quartermaine put the woman in his truck and then directed traffic around the burning vehicle until fire-
fighters arrived.
The motorist, who declined to be named, said she was driving her mother's car and had no idea it was on fire until Mr Quartermaine arrived.
The fire disrupted the flow of traffic leaving the city after Saturday night's big Eagles concert, as the motorway was closed for a short time while firefighters extinguished the blaze, police said.
The fire was out and the motorway reopened by 11am, when a tow truck removed the charred remains of the vehicle.
