Staff noticed people waited for a particular product to be on special and stocked up when one of their favourites was discounted.
Specials were chosen to fit in with seasonal or topical trends, Schepen said: "When the tomato season kicks off and we start getting good supplies [those are featured] or some products to match an upcoming sporting match."
Rich said suppliers and retailers worked together to plan out over a number of months what specials would be and when. "Retailers have the greater amount of control but it's a relatively collegial process."
Rich said discounts were usually offered by the supplier to the supermarket. "More often than not it's the supplier who makes some concessions. Sometimes it's a supermarket loss leader that the supplier isn't funding."
Schepen said Weekend Windback specials were never sold at a loss. Progressive Enterprises staff worked to get good prices from suppliers.
"Behind the scenes, Weekend Windbacks work the same as regular specials.
"Our buyers work hard to secure the best deals possible and those savings are passed on to our customers through lower prices.
"The theory of buying in bulk to save works for a retailer, too. When we purchase more products from our suppliers, we can get a better price, which we can then pass on to our customers."
Rob Chemaly, general manager retail of Foodstuffs Auckland, which covers Pak'n Save, New World and Four Square stores, said: "Products that fly off the shelves during a promotion include canned goods, baby products, meat, hot chickens, bread, biscuits, soft drinks and confectionary."
Pierre van Heerden, of Sanitarium, said increased sales through special promotions were hard to predict.
"It varies enormously based on price and what else is on promotion. In order to attract customers, everyone gets involved in some form of promotion. It's just the level to which one sells on promotion."
Watties said: "We participate to help drive sales volumes ... on occasion there will be the opportunity to encourage consumers to try a new product."
Managing director of Nielsen Consumer Rob Clark said there was evidence consumers were less brand loyal in some categories. "There's a phrase used: Poor people need good prices and rich people love good prices."
Interestingly, discounts are reflected in national food statistics. Chris Pike, prices manager of Statistics New Zealand, said data collectors took the price the day they visited, regardless of whether it was a promotion. In August, 24 per cent of prices fell and much of that was because of specials.