A security system computer glitch is being blamed for the doors opening and the lights coming on at a Hamilton supermarket, allowing a number of people to go in and help themselves to stock yesterday.

The central city Pak 'n Save was open for more than eight hours from 1am until someone rang police saying people were leaving the shop with "truckloads of groceries", the Waikato Times reported.

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.

An embarrassed Mr Miller said he did not know how much stock was taken but he was delighted that 12 people had used the self-service tills to pay for items.