The Commerce Commission is now involved in a FoodStuffs investigation after a pricing blip saw 70 North Island supermarkets overcharge customers this week.
The supermarket giant says it is investigating the technical issue that occurred yesterday between 7am and 12.30pm at Pak n' Save, New World and Four Square stores.
A Foodstuffs spokesperson apologised to affected customers and said the company was "investigating the data to accurately determine the best action to put this right".
"We again sincerely apologise to any of our customers who shopped with us in affected stores.
"We work hard every week to bring New Zealanders value and hundreds of great specials, the delay and the mismatch between shelf and checkout price has not been the experience we strive to deliver for our customers, and we are committed to making this right."
Foodstuffs is yet to confirm how many customers were affected, or how much they were collectively overcharged.
Some customers were undercharged, the company said, but "will of course not be asked to pay any difference".
"As soon as we're in a position to confirm how to receive any appropriate refund we will communicate this via our brand websites. We are also engaged with the Commerce Commission to ensure they have the relevant information."
A Commission spokesperson confirmed they had received "one consumer complaint potentially related" to the pricing glitch.
Foodstuffs had not reported the incident to the commerce watchdog.
The spokesperson said the company was not required to self-report to the Commerce Commission, though some businesses did when issues arose.
"If a mistake is made, businesses should ensure consumers are compensated and take immediate steps to ensure that the mistake is not repeated."
On Monday, a technical issue led to nearly a quarter of "new special prices" failing to register at Pak'nSave, New World and Four Square checkouts.
The issue occurred at 7am and Foodstuffs resolved the matter by 12.30pm.
One Pak'nSave Silverdale shopper said she had specially gone to the store to pick out a wide range of specially discounted products.
However, she only noticed a printed out sign saying "our prices may not be scanning accurately" once at the checkout and halfway through scanning her "weekly shop".
The woman asked the attendant what the sign meant and how would she know if she'd been overcharged.
But the attendant told her it was only her second day on the job and she didn't know about the pricing issue.
"I thought, 'Are you kidding me'.
"If people are on a budget and have been going through getting all the specials but they are not being rung up as specials - it doesn't sit well with me.
"Generally, consumers should be able to trust that the price displayed on the shelf is the price they will be charged at checkout."
Another Countdown and New World shopper said they were "regularly finding the price on items posted in the isle is higher at the till".
The shopper said it often occurred once or twice a month, meaning they had a "sharp eye on price[s]" when shopping.