Supermarkets are discounting drinks loaded with sugar and salt more often than the healthy alternatives to pull shoppers through the doors.

A study by Otago University recorded 1487 discounted drinks from four Wellington supermarkets over four weeks.

They found 85 per cent of the discounts were for unhealthy products such as soft drinks, sports beverages, flavoured waters and cordial.

Just 15 per cent of the discounted drinks were considered healthy including water, plain reduced-fat milk or reduced-fat soy drinks.

One of the study authors, Louise Signal, described the recent increase in obesity as "alarming" and said policy makers were looking for solutions.

"We are interested in the role of the environment in promoting healthy diet, and supermarkets are a key part of this environment," she said.

The Obesity Action Coalition executive director Leigh Sturgiss said supermarkets often used sugary soft drinks as "price leaders" to draw people into stores. "We seriously need to look at what food and drinks are available to our children, and how expensive they are."

"The evidence is clear that sugary drinks are playing a part in the increase in child obesity."

Sturgiss suggested a sugar tax, similar to the tax on smoking, with the money gathered going back into health education.

One supermarket company said it was taking steps to educate children on good nutrition.

Steve Anderson, chief executive for Foodstuffs South Island which owns the New World and Pak'n Save supermarkets, said: "We take our obligations very seriously".

He cited programmes like Food for Thought, which he said had taught 115,000 primary age children about food and lifestyle choices.

Calls to the other major supermarket chain, Progressive Enterprises, were not returned.

Earlier this month an international study found New Zealanders were getting fatter because they ate too much.

To return to the average weights of the 1970s, children would have to cut their food intake by about 350 calories a day - a can of fizzy drink and a small chocolate bar - according to Auckland University of Technology nutritionist Elaine Rush. Adults would have to cut 500 calories a day - the equivalent of a large burger.