The Warehouse is among retailers accused of misleading customers by selling items marked "not for individual sale".
Many stores - usually independent outlets - make extra money by breaking up multipacks bought at a discounted rate and selling the contents for the standard single unit price.
The practice is often applied to soft drinks, chocolate bars and bottles of beer.
Retailers claim savings are passed on to consumers, but watchdogs and manufacturers argue shoppers would get a better deal by bulk buying at supermarkets.
We discovered several inner-city Auckland dairies selling cans of Coca-Cola and Sprite marked "not for individual sale" for between $1 and $1.70.
That's a 50 per cent mark-up compared to buying a 12-pack of Coke from Foodtown for $6.99 - 58 cents a can.
Mid City Newsagents sold single cans of Coke for $1.50 and said they broke up multipacks for convenience.
"I buy packs of maybe 20 or 30 but break them up so it's easier for the customer to buy one, otherwise they have to buy the pack of 30," said the manager, who would not be named.
"If I buy one I would have to sell it for $2 or more."
Even The Warehouse is getting in on the act.
The chain's downtown Auckland store was charging $18.49 for 24-can multipacks of Coca-Cola - about 77 cents each - but $1 for a single can, despite it carrying the no resale label.
The Warehouse did not respond to requests for comment before deadline.
Foodtown sells six bottles of Heineken for $13.99 - $2.33 each - whereas Sai Superette and Liquor on Kingston St sells single bottles for $3.90.
Consumer New Zealand said the practice was not illegal.
"When you buy from local dairies you pay a premium for the convenience and the fact you're often buying a single item rather than a pack," said Consumer editor David Naulls.
"It's more than likely you'll get a cheaper deal by shopping at the supermarket, particularly if buying in bulk."
New Zealand Food Safety Authority spokesman John Van den Beuken said selling individual items from multipacks only breached the Australia New Zealand Food Standards Code if they did not carry the appropriate nutritional information.
Coca-Cola Amatil New Zealand spokeswoman Aimee Driscoll said the company received "regular" complaints from consumers about the resale of bulk items.
While not illegal, Driscoll said it wanted to discourage customers from buying from multipacks designed to offer value to supermarket shoppers.
Cadbury spokesman Daniel Ellis said most retailers did the right thing, but the company frequently made spot checks. He said reselling products marked not for individual sale was misleading because multipacks were made for consumers who preferred to buy in bulk.
"That is how we'd expect consumers to be able to purchase them."