Manufacturers who shrink their products while leaving the price the same risk taking a big hit as shoppers become increasingly wise to the practice, says Consumer New Zealand.

Iconic New Zealand brand Tip Top is the latest manufacturer to treat consumers "as though they are a little bit silly" by cutting the size of some of their traditional two-litre ice cream packs to 1.6 litres but not cutting the price.

Flavours Goody Goody Gum Drops, Jelly Tip, Monkey Business and Gone Fishin' have been shrunk, with the company justifying the change by saying the smaller tubs were packed with more "exciting bits".

The changes were made because other flavours had been "carrying" the cost of these flavours for a while, it said.

If the size of these packs was not cut, the price of the more traditional flavours would have had to increase.

Earlier this year, Cadbury reduced the size of its popular 150g and 250g bars of chocolate by about 20 per cent and started using palm oil as part of a cost-cutting exercise.

A consumer backlash saw them backtrack on the palm oil decision but not the downsizing.

Other products to have been given the "shrink" treatment include some tinned goods, breakfast cereals, cans of beer (from 375mls to 330mls), pre-pack Yummy apples (from 2kg to 1.5kg), and the 1kg block of cheese (some cut to 900g).

Consumer NZ testing manager Hamish Wilson told NZPA the practice of downsizing packaging was widespread but risky for the manufacturer.

"Consumers do react to all of this. There was a very strong reaction to the Cadbury change," he said.

"Their competitors appear to have made a considerable amount of hay out of it.

"The chances are there will be a quite a backlash to the Tip Top decision and people will look at alternative brands and products."

To say there were now more "goodies" in the new packs was simply putting "some spin" on what was an effective price increase, Mr Wilson said.

"The fact is, it is the same amount of goodies in a smaller pack. They are treating us like we're a little bit silly.

"It's a price increase and that's that."

Mr Wilson said the practice of downsizing was one reason why Consumer NZ was "so keen" to see unit pricing, or price by weight, made mandatory.

"The marketers will pull all the tricks out of the hat that they can to keep us confused.

"Unit pricing is the only way you can really compare prices."

Tip Top managing director Alastair De Raadt said the company would look at consumer reaction "very closely" but reversing its decision was "very unlikely".

Daniel Griffiths, acting manager at Statistics New Zealand's pricing unit, said both shelf price and weight were taken into account when Food Price Index figures were being collated.

- NZPA