A supermarket mega-giant could be forced to compensate customers ripped-off by a technical glitch

About 70 North Island Foodstuff stores accidentally overcharged people for some groceries meant to be on special yesterday.

A consumer law expert says it's illegal - and Foodstuff's could be forced reimburse customers by the Commerce Commission.

Nikki Pender says it will ensure this doesn't become a casual mistake which slips by.

He says to individual consumers it may not seem like a big deal... but you can imagine a lot of money in total was overcharged.