Shoppers who took advantage of a supermarket that accidentally opened with no staff could be considered part of a "real life candid camera" or patients in a "mad experiment" that explored morality, a professor of religion says.

About 24 people cruised the aisles at Hamilton's Mill St Pak 'n Save in New Zealand on Good Friday morning after a computer glitch incorrectly opened the doors at 8am without a checkout person in sight.

About half paid for their groceries using the self-scan service, but that stopped working when someone scanned alcohol, which requires a staff member to check a customer's age before the system is unlocked.

Police were alerted at 9.20am by a report of people leaving with "truckloads of groceries".

Victoria University's professor of religious studies, Paul Morris, said the Easter-time ethical test placed customers in a tricky situation. "It is like real life candid camera where people are clearly faced with this series of moral dilemmas."