Why Kiwis have gone 'sticker mad' over Smeg supermarket promotion

A set of knives has turned Kiwis into "sticker mad maniacs".

New World's Smeg knife promotion has proved to be their most popular promotion ever. 

The supermarket awards shoppers a sticker for every $20 they spend. After collecting a certain amount, shoppers can redeem the stickers for a selection of Smeg knives or a stylish knife block.

Shoppers have gone mad as knives and knife blocks prove to be out of stock, even though the promotions says it will only go as "long as stocks last". 

Consumer NZ believes New World could have "done better". 

The Projects Jesse Mulligan says the set of knives has turned Kiwis into "sticker mad maniacs." 

So why are Kiwis going mad for knives?

Clinical Psychologist Dougal Sutherland told The Project that while humans like to think they are evolved they can't help but get excited over a prize. 

"Just as you might give a kid a sticker chart for getting up in the morning or brushing their teeth we respond as adults to sticker charts as well."

"Even if you can just go and buy it it's like 'it's a reward it's great, I really want it'." 

Head of marketing at Auckland University Bodo Lang says exclusivity is another factor in the madness. 

"Sometimes the sense that this item is only available for a short period only through this particular channel, I think we saw exactly that happening with the knife blocks." 

Lang also says there's a rational aspect, the consumers feel as if they are getting more than they have given. 

Over the time of the promotion, customers would have had to purchase $5000 worth of groceries to get the full knife set. 

Watch the full video above.