A Hawke's Bay study has for the first time provided proof of an obesity epidemic in New Zealand children -- and shows that while Maori and Pacific Island children are the worst affected, European children are catching up fast.

Researchers found the proportion of children classed as overweight over 11 years almost doubled from 11 to 20.9 per cent, and obese children more than tripled from 2.4 to 9.1 per cent.

The study, co-authored by Hawke's Bay Hospital paediatrician David Barry and published in the Journal of Paediatrics and Child Health, compared just under 900 Hastings and Havelock North children aged 11-12 in 1989 with a similar group in 2000.

The children were compared by body mass index (BMI), which is calculated by dividing weight in kilograms by height in metres squared.

"This tells us 30 per cent of children are overweight or obese, and there's almost a 1 per cent per year increase in BMI," Dr Barry said.

While Pacific Island and Maori children still had the highest BMIs, European children were catching up because their obesity rates had increased faster over the 11-year period.

Doctors have long been alarmed by anecdotal evidence suggesting New Zealand children are packing on weight, but the Hawke's Bay study is the first peer-reviewed, scientific proof.

The researchers also compared New Zealand obesity rates with those in six other countries, including Britain and the US, and found New Zealand children were at the heavy end of the global obesity epidemic.

"That means we've got to join the world effort against obesity, and come to grips with the problem well before these children become adolescents -- then it's very hard to control," Dr Barry said.

While the answer was simple -- eating less and healthier food, and exercising more -- putting it into practice was often difficult.

"On an individual level it needs lifestyle changes, like trying to get people to exercise 30 minutes a day, stop snacking on fast food and soft drinks and watching less TV," he said.

The paper also suggested national initiatives such as healthy food policies in schools and legislation to protect children from television marketing of high-fat, high-sugar foods.

The research was funded by the Hawke's Bay Medical Research Foundation, the Health Research Council of New Zealand and the Asthma and Respiratory Foundation, and was part of a world-wide study into the prevalence of asthma.