For instance, one Masterton consumer complained to Stuff last week that the milk that was usually cheapest at their local Pak n’Save supermarket had disappeared, seemingly replaced by a new low-carbon label, Simply Milk, which was $4.27 for two litres.

To get an idea of how much more it was, the nearest rival Countdown was selling an in-house brand for $3.70.

But the customer’s favourite, Value milk, was available at other Pak ‘n Saves. And not only was it available, it was at difference prices: $3.55 at the Upper Hutt branch and $3.69 at the Lower Hutt.

So why the price differences within the same brand? For its part, Countdown said its prices were determined nationally.

But Foodstuffs, which owns Pak ‘n Save, as well as New World and various other brands, is a co-operative which means branches can make independent decisions.

“As our stores are owner-operated, each store has the flexibility to adjust the range and price of many products locally, enabling a tailored offer that delivers great value on the things that are most important in their local community,” Foodstuff spokeswoman Antoinette Laird said.

These decisions can mean that a supermarket might stock brands that are more popular with their local community than with another, she said.

“Other qualities that motivate which milk to select include the size and shape of the bottle and, of course, price.”

That answer might not fully satisfy the consumer, however, who said not having the budget brand in her store was pretty harsh for some customers on low incomes.

All this seems to suggest that if there can be price differences within even the same brand of supermarket, it pays to shop around, and one of the keys to doing this is to go off the unit price per weight.

When supermarkets do give these, it shows that, for example, a two litre bottle of milk, is more cost effective than one litre of the same brand.

For example, two litres of Anchor Blue at the Masterton Countdown this week was $4.81, or $2.41 a litre. But the price for a one litre bottle is $2.98.

Buying bigger or in bulk can transcend this problem, for those who can afford it.

International bulk retailer Costco is coming to Auckland but it will probably only be of use to those who live within driving distance.