The Commerce Commission is accusing a supermarket in Auckland overcharging at the checkout.
The Commission has charged Pak'nSave Mangere with 12 counts of breaching fair trading laws between June and October 2018.
It claims the supermarket, trading as Kennedy's Foodcentre, charged higher prices at the checkout than the promotional prices on the shelf or online.
Commission staff sent mystery shoppers to check advertised prices against those at the till.
The Pak'nSave brand is owned by Foodstuffs which licences the operation of supermarkets to franchisees.
The commission said promotional pricing was advertised online via the Pak'nSave website on a Mangere-specific webpage on price labels and signs in the store.
"The charges arose after Commission staff conducted mystery shops to check advertised prices against those charged at the till," the commission said in a statement.
"Alleged price discrepancies identified by Commission staff were raised with customer service staff at Pak'nSave Mangere immediately following the mystery shops.
"Commission staff then returned the next day and re-purchased the products for which they had identified price discrepancies, to check whether the price discrepancies were still occurring."