Consumer New Zealand is urging people to think carefully before going wild at Boxing Day sales.
Chief executive Sue Chetwin said the balloons and bright lights of sales day were enticing, but shoppers should research products carefully before buying, to make sure they were the right things.
"What can happen that you've got something heavily discounted but discount is soon eaten up with the interest rate that you're paying on the credit card if you don't pay your card off in full," Ms Chetwin said.
"We know some people get a bit of a shock when their bills come in January and February just how much they have spent pre Christmas and then in the Boxing Day sales."
Ms Chetwin said retailers were not obliged to refund or exchange products unless they were faulty.
Meanwhile, Retail New Zealand believes brick-and-mortar stores will likely be swapped out for smaller showrooms due to the rise in online shopping.
Retail NZ general manager of public affairs Greg Harford said customers were increasingly wanting to buy products online.
He said there would always be demand for physical stores to show items, but in the future retailers would probably focus more attention online.
"You could be seeing individual stores moving perhaps to a smaller footprint, you could be seeing them operating more as a showroom where customers go and see things before then going and ordering them online and having them delivered to their homes," he said.
Mr Harford said rules that came into effect in October, forcing overseas retailers to pay goods and services tax on purchases shipped to New Zealand, would even out the competition for local retailers.