Kate Malster was paying for her groceries when she saw a woman with a full trolley running full speed towards the exit of the Greenlane Countdown.
Employees from Countdown were shouting "stop, don't do this" to the woman before she exited the store.
Malster heroically chased the woman into the car park of the Greenlane supermarket, attempting to grab the trolley and telling her to stop before she escaped.
The woman reportedly pushed the trolley to two other people that were waiting at the rear-end of the getaway car and unloaded the goods.
"I have never seen food go so fast from a trolley into the back of a car... It was absolutely orchestrated so this definitely isn't their first time," said Malster.
"I had to back out when I did as I am only 10 weeks post spinal surgery," said Malster.
"The countdown staff offered for me to come in and gave me a box of chocolates and water while I tried to calm myself down.
"I wasn't expecting that in my long weekend," she said.
A police statement said a group of three shoppers left Countdown Greenlane on Great South Rd and did not pay for their groceries about 2pm yesterday.
Police are making inquiries into the incident and a report of the getaway vehicle breaking down on the Southern Motorway.
"They left in a vehicle and the incident was reported to the police. A short time later a Countdown customer reported seeing the vehicle the group was driving in broken down on the Southern Motorway, Northbound near Khyber Pass," the statement said.
A Countdown spokeswoman said declined to comment.