Consumer NZ wants people to be aware of their rights when returning and exchanging gifts.
The Consumer Guarantees Act does not require a retailer to give a refund if a person buys something and then changes their mind about it.
But Consumer NZ spokesperson Jessica Walker said some stores had generous returns policies.
Consumers would however need either a receipt or proof of purchase, she said.
"You can't just rock up at a store and say 'hey, I think that somebody bought this from here and I want to return it or exchange it please' because obviously that's not fair on the retailer."
The consumer watchdog also warned people against adding extended warranties to their purchases when hitting the sales this season.
The Consumers Guarantees Act afforded people enough protection, Walker said.
"With an extended warranty, most of the time you're actually getting less protection or equal protection to what you've already got under existing law ...
"That means that if your good develops a fault pretty quickly, then you are entitled to get it repaired, to get an exchange or, if the fault is catastrophic, to get a full refund - it should last longer than the manufacturer's warranty as well."
Walker said if something did not work, consumers were entitled to a refund, even if it was bought on sale.
Shopping malls in Auckland were busy today with people searching for Boxing Day bargains.
Westfield Newmarket was evacuated for a time this morning when a fog cannon was accidentally set off. Fire and Emergency sent two fire trucks to the scene.
Footage on social media also showed long queues to enter Dressmart in Onehunga.
And traffic around Westfield Manukau was heavy as shoppers tried to find parks at the precinct.