A notice on the door to Super Liquor Anderson's Bay in Dunedin.

A Dunedin liquor store has been hit with a record 21-day off-licence suspension after booze was sold to a minor.

Super Liquor Anderson's Bay has been suspended for selling alcohol from Monday August 10 until August 31 following a failed controlled purchase operation, with the decision understood to be the longest suspension in Dunedin's history.

That operation involved a pair of 16-year-old volunteers, who did not have identification, buying buy alcohol on Friday, October 24, about 9.30pm.

Super Liquor Anderson's Bay in Dunedin has been slapped with a record 21-day closure after it failed a controlled purchase operation.

They took a four pack of RTDs to the counter, and were not asked for their age or for any identification by the duty manager.

Before the Alcohol Regulatory and Licensing Authority on July 13, it was stated the duty manager had suffered a head injury - knocking her head on the garage door - shortly before the controlled purchase operation.

The Authority was told the duty manager "felt very light-headed, dizzy and confused and it was in this state that she served the volunteers".

The 19-year-old duty manager, who is six and a half months pregnant, was later driven to hospital by her mother.

However, Sergeant Ian Paulin said it was a small graze and not a significant head injury, and there was no documentary evidence supporting her visit to hospital.

He noted it was the second failure of the Anderson's Bay premise in two years, while the director and shareholder of the company which owned the store, Patricia and Kenneth McCarthy, also were directors and shareholders in several other companies.

There had been five previous controlled purchase operation failures by these companies in Dunedin since December 2007.

Patricia McCarthy said her company had zero tolerance for managers committing any breaches of the Act.

She confirmed that at the stores a till prompt asked for a date of birth before a sale proceeded; however, she conceded that could be overridden.

After the last failed controlled purchase operation, the Authority warned any future transgressions would be dealt with on a "cumulative basis as this would involve systemic problems".

In his decision, Judge JJ Weir suspended the off-licence for the Andersons Bay Rd store and suspended the duty manager's certificate for one month beginning on Monday, August 10, 2015.

McCarthy said her company, duty manager, and lawyer were yet to receive the decision in writing, with the business opening as normal on Monday morning until she was contacted by a local liquor inspector.

"We are going to appeal the decision," she said.