Research from PriceSpy showed 15 percent of all products listed on its site in last year's Black Friday campaign were "fake sales".
Spokesperson Tim Provise said it was a common tactic that consumers needed to watch for, as big markdowns appeared online and in stores.
"You might think that you're getting 50 percent off, but you're not, when compared to the price 30 days ago, it's more like 20 percent," he said.
"While many retailers offer genuine discounts, our research also shows a concerning increase in fake sales during major events like Black Friday. We advise consumers to be particularly diligent and conduct thorough pricing research before making any purchases across this year's sale season."
Consumer NZ echoed PriceSpy's comments about shoppers needing to be on high alert over the coming months.
Campaigns manager Jessica Walker said high-pressure sales tactics were prevalent online and in-store.
By taking time out to research purchases thoroughly, she said shoppers could avoid making unwanted impulse buys.
"These pressure tactics are used by online retailers and in-store for a reason," Walker said.
"Especially in online shopping you'll get these pop-ups that will say things like 'selling fast' or '10 people looking at this item now'. Those kind of things are designed to get you to part with your money quickly, so we really recommend that people take a breather.
"First of all, establish the usual selling price and don't fall for the hype."
Consumer NZ research interestingly showed the highest-priced items were not necessarily of the best quality.
PriceSpy's data showed 16 percent of products it monitored actually saw price increases during last year's Black Friday period, compared to normal shopping days.
"There are some significant discounts to be found but not all deals are as beneficial as they first appear," Provise said.