Pak 'n Save was the cheapest for 30 items, including budget bread and milk.

Pak 'n Save has been crowned the country's cheapest supermarket in Consumer NZ annual price survey.

The supermarket claimed top spot in all seven centres surveyed, a place it has held almost unchallenged since 2001.

Mystery shoppers bought from a list of 30 items. They excluded wine and meat, fish, or poultry which would have been subject to questions about quality.

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Pak 'n Save was followed by Countdown and New World, which is also owned by Pak 'n Save's owner Foodstuffs.

Consumer NZ chief executive Sue Chetwin said a feature of this year's survey was the significant number of products purchased at Countdown that were on special, giving it the edge on New World for selected items.

Between 37 and 50 per cent of all the items bought at Countdown were marked as specials, "a pattern we've seen in previous years".

Chetwin said price promotions had become a regular feature in the grocery trade, with market research firm Nielsen finding that the proportion of groceries bought on special each year was as high as 59 per cent.

With food being the second biggest household expense after housing, and the industry being dominated by two major players, Chetwin called for regulators to keep an eye on promotional pricing.

"While special offers can reduce the weekly grocery bill, if an item is routinely discounted, you have to ask how 'special' is the special price?"

Stores tempting customers with specials must be offering a genuine deal, or risk breaching the Fair Trading Act.

"The two big chains keep a close watch on each other's prices. Regulators need to keep regular tabs on them."