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Honest shoppers and opportunists took advantage of the accidental opening of Hamilton's biggest supermarket over the Easter break.

There wasn't a checkout operator in sight when Mill St Pak'nSave opened at 8am on Good Friday because of a computer setting error.

The store was supposed to be closed for the public holiday, but the doors opened and the lights came on as usual because the security system was set incorrectly.

About 50 shoppers walked through the doors and took advantage of the unexpected opening - 12 of those showed their honesty by paying for their groceries using the self-service tills, said owner Glenn Miller.

"They were just happy to be able to buy something.

"One lady was here by herself for about 20 minutes and paid. She didn't seem to notice no one was here," he said after reviewing the security footage.

A further 12 shoppers stocked up their pantries without paying and the remainder abandoned their trolleys when they realised the store was unmanned, said Miller.

A regular customer called police from the store to alert them that people were leaving with groceries.

There were several people with trolleys and cars full of groceries when officers arrived at 9.20am, said Sergeant Guy Callahan.

"We weren't to know who had paid and who hadn't. Our main priority was to get people out," he said.

The shoppers' mixed behaviour was typical, said clinical psychologist Dr Clare Calvert.

"When people are presented with an opportunity like that there's always going to be a proportion of people who are going to take the opportunity."

Difficult financial times was probably one of the biggest factors why people took opportunities like this, she said.

"It's like when cashpoints give out extra money when there's a computer error. People think it's the same with supermarkets. They think, 'I need the money more than they do'." But many would always do the right thing, she said.

Miller said the cost of the stolen goods was still to be estimated but he was delighted 12 people had used the self-service tills to pay for items.

Another shopper called the store yesterday "to say they were innocent and didn't take anything", he said.

Callahan praised those who paid. "Good on them. It just goes to show there are some honest people out there. Those who didn't pay should do the honest thing and come forward and pay for their groceries."

Police are working with supermarket staff to review the security footage and identify offenders.

Miller suspected the fault was linked to a command cancelling the normal opening time for the day. "It wasn't set up properly and we hardly tested it because we're always open."

The issue with the computer system has since been sorted out and the store would be closed today as planned. He wanted those who hadn't paid to cough up and he would donate the money to the Christchurch Earthquake Appeal.