Opportunists took the chance to stock up their pantries when the doors to Hamilton's biggest supermarket opened for about eight hours early yesterday without a checkout person in sight.

A security system computer glitch is being blamed for the doors opening and the lights coming on at Pak 'n Save in Mill St about 1am yesterday. Sergeant Guy Callahan said police were alerted at 9.20am by a member of the public saying loads of people were leaving the shop with "truckloads of groceries".

"She said people were coming out the store with shopping trolleys full of stock and yet there were no people manning the checkouts and no-one was even in the store."

Callahan said three police units were sent to investigate the situation.

"When we arrived there were cars leaving all over the place. There were some people in the shop at the checkouts legitimately purchasing items [using the self-service facilities] and others in the store.

"We couldn't tell who had paid and who hadn't and so our first priority was to close the supermarket and contact the owners."

Owner Glenn Miller said he suspected the fault was linked to a command cancelling the normal opening time for the day. Usually the supermarket opens at 8am every day but the shop was meant to stay closed for Good Friday.

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

And while Miller is a little embarrassed by the incident he is appealing to those who left without paying to return and cough up a bit of cash.

"It would be really nice and if they do I will definitely contribute all of the money to Christchurch [earthquake appeal]."