Caitlin, who said she was told to wear her usual clothes and not to try to look older than she was, provided her passport as identification when making the alcohol purchase.
There were eight other customers and four staff in the store at the time, she said.
The owner-operator of the Four Square, Tim Baker, told the Herald the decision was being appealed.
"We take our responsibility as a retailer of alcohol very seriously," he said.
The Four Square argues an anomaly in the point of sale system occurred when Caitlin purchased the cider and the checkout operator instead bypassed the system.
When an age-restricted item is scanned a pop-up screen appears and requires a date of birth to be filled out for the sale to go ahead.
However, an authorised staff member can bypass the system instead of filling out the system.
When Caitlin appeared at the counter with the cider, the operator scanned it and asked for her identification when he noticed she looked under 25.
Caitlin handed over her passport to the operator and he filled out the system with her date of birth, which shows she was born in 2002.
In evidence, the operator said he thought he entered the date of birth incorrectly, deleted it and entered the date of birth again.
It was at this point, he said, the system failed and the screen went blank for two or three seconds.
Aware of having to serve other customers, the operator decided to rescan the cider and bypass the authorisation system with his pass.
The operator admitted not knowing who the on-duty manager was at the time.
The same licensee previously failed a controlled purchase operation in January 2017.
Because the operator was not a manager but held the authority to bypass the system, because he did not know the on-duty manager and because of their previous failure to sell alcohol to an underage person, the licensee faces a 10-day suspension period.
