Black Friday has become one of the biggest sales days around the world but new research shows some discounts may be dud deals. 

Pricing and comparison service PriceSpy says nearly one in five products it listed saw a price hike on Black Friday, a marked increase of 7 per cent compared to last year. 

More than one in 10 products offered a fake sale, where the price of an item was hiked ahead of sale day to make the Black Friday discount look better than it was. 

"The bigger Black Friday is becoming, the more we have seen a spike in the number of fake sales being offered, which is really disappointing to see," said Liisa Matinvesi-Bassett, New Zealand country manager for PriceSpy. 

Computers and accessories, mobile phones, games and consoles, and speakers and headphones saw the biggest number of price hikes, even though the same product categories also saw significant price drops. 

For example, 25 per cent of products listed in PriceSpy's computers and accessories category saw a price hike on Black Friday. At the same time, 33 per cent of products in this category saw a price drop. 

Matinvesi-Bassett said the research shows shoppers need to dig deeper and look at how much the item they are looking to buy has been sold historically to see if a Black Friday -or any - discount is true. 

PriceSpy research showed 38 per cent of products saw a price drop on Black Friday 2021, and 19 per cent dropped by at least 10 per cent or more, offering an average saving of -21 per cent. 

It said the biggest Black Friday savings this year came from fitness watches, PS5 games and electric toothbrushes, which saw price index changes of between -18 to -29 per cent according to its price index data. 

"Whilst sales days like Black Friday can help boost sales opportunities for retailers who have had a very difficult time these last few years, to really earn the trust of shoppers who are being sold the dream of getting a real bargain, fake sales need to be avoided and pricing strategies need to be more transparent," said Matinvesi-Bassett.