They conducted the study to inform the Advertising Standards Authority's review of the code of advertising food to children. They recommend using the WHO system.
"Given the recognised weak nutritional standards employed by industry for defining healthy foods and because many child-oriented food marketers do not participate in self-regulation, the new children's code ... should be [independently evaluated].
"If the revised voluntary code still proves ineffective in reducing New Zealand children's exposure to the marketing of unhealthy foods and drinks, additional policy and regulatory actions will be necessary."
The voluntary Health Star Rating system was developed by the Government with the food industry.
The major food category differences between the three systems - in breakfast cereals, fruit bars, fruit/vegetable juices and dried fruit - "appear to be due mainly to the different weighting that each system gives to sugar, with HSR in particular ... more lenient in classifying more high sugar products as eligible."
Association of Advertisers chief executive Lindsay Mouat said the WHO criteria were deliberately high.
"Industry does not dismiss the WHO criteria out of hand but by WHO Geneva's own admission, they are 'tough and aspirational' which is why you don't see them being adopted.
"Critically, you cannot ask industry to reformulate their products, take out fat, salt and sugar and then forbid them from marketing the 'better-for-you' version, which is what the WHO classification does.
"We [all want to induce] behavioural change; that means slowly weaning consumers on to healthier options."
The Food and Grocery Council declined to comment on the research, referring the Herald to its submission on the code review, which says the code is working well.
The code reflects the "high standards for responsibility by the industry that ensure parents' and children's trust and protection for the benefit of all of the community".
The council advised reducing the age group to which the code applies to under 12, from under 14 at present.
Food Safety Minister Jo Goodhew defended the star-rating system.
She said: "The Health Star Ratings system was not designed as a tool to restrict marketing to children, but as a way for consumers to easily access nutritional information, compare similar packaged products, and to encourage producers to reformulate their products to support healthier food choices."