Firefighter Murray Pike, who was helping to keep neighbouring cars cool and safe with a hose, said luckily someone rang the fire service as the car was parked between two other vehicles in the carpark.
"It could have easily spread to the other cars," he said.
Shannelle Aldridge was inside Countdown when people ran inside to say there was a vehicle alight in the carpark.
"I saw this silver Peugeot. It was on fire. When the firefighters lifted the bonnet there were flames and lots of black smoke coming from the engine. There was a big hole in the bonnet," she said.
Michael McRae said he had seen cars ablaze before and thought it could have been a fuel leak dripping on to plastic fittings, which could have caused the fire.
Another spectator, Leanne Wright, said it was a frightening experience as she pushed her toddler in a trolley towards their car, four down from the burning Peugeot. "It's just one of those things but it could easily have got out of control."