A supermarket in Blenheim has lost its alcohol licence for six weeks for selling to minors.

Countdown Blenheim, at Arthur St, failed two controlled purchase operations in June and September last year.

It was the fifth time the store had sold to a minor since 2006.

The 42-day suspension starts on April 26.

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A 17-year-old boy attempted to purchase a bottle of wine as part of a police booze sting in Marlborough on September 23 last year.

The supervisor looked at his identification, which showed he was 17, and sold him the wine anyway.

Countdown Blenheim failed a similar sting in June 2015 in which a supervisor did not enter a 17-year-old boy's date of birth into the till computer and the boy left with a box of beer.

Countdown lawyer Duncan McGill said human error was to blame at a hearing at the Blenheim District Court on March 10.

The supermarket accepted it had failed in its obligations under the Sale and Supply of Alcohol Act, but had taken steps to improve its alcohol policy, he said.

Countdown operations manager Jo McNaught​ said checkout supervisors were usually required to ask anyone looking under 25 for identification, but this was raised to 30 after the sting.

All staff had completed retraining from a qualified liquor licence trainer and would be required to complete a refresher course every three months.

Three staff members were fired after further internal disciplinary measures, McNaught said.

Countdown was making efforts to act responsibly, but the lack of physical assistance given to supervisors to help calculate the minimum birth date was surprising, the decision said.

Supervisors should have managers' certificates given the high volume of alcohol sales each day, it said.

The Alcohol Regulatory and Licensing Authority said it was concerned to learn some checkout operators were as young as 16-years-old.

The authority referred to Lynfield Countdown's failed police sting in 2006 that said Countdown was considering a policy for checkout operators to be at least 20-years-old.

Penalty guidelines suggested five days' suspension for supermarkets, but the store's earlier failures were an aggravating factor.

Credit was given for admitting fault, a five-year gap between failures, and the measures taken to avoid failures in the future.

Countdown general manager Brett Ashley said his team were extremely disappointed to have failed the police stings.

"We apologise to our Blenheim customers for the inconvenience this suspension will cause them, and for getting it wrong. We are a long-standing part of the Blenheim community and it's important to us to sell restricted products responsibly," he said.

The alcohol department would be "blocked off" during the suspension, and staff would not be affected.

Till software to calculate the age of a customer would be installed later in the year, he said.

In 2011, the Countdown chain had 21 alcohol licence suspensions around New Zealand.

Sergeant Peter Payne, of Blenheim, said the decision to suspend their licence for 42 days was reasonable.

"I don't know of any other supermarkets with five suspensions," he said.

Selling alcohol to minors caused significant harm to young people and the wider community, Payne said.

Police had more stings planned for later in the year, he said.

A police sting checked eight off-licence premises in Blenheim, Renwick and Picton last week.

Marlborough District Council licensing inspector Karen Winter said two 16-year-old boys were denied sale at all eight shops.

She would not say which premises were targeted.