A leading obesity researcher says changes to the way the Canterbury District Health board identifies and treats obesity in preschoolers will be a wake up call for parents.

From the first of July, Canterbury families with four-year-olds that qualify as obese at their "before school checks" will be referred to a range of programmes that encourage healthy eating and exercise.

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Otago University research fellow Rachael Taylor said that could be confronting for many parents who don't recognise their child is overweight.

"In particular at that age, in pre school age, obesity really isn't viewed as a problem in a child by parents, unless they're being very heavily bullied."

Ms Taylor said some young kids will grow out of being overweight, but unfortunately at least half of them won't. 

She said education is currently the most effective tool doctors have for reducing obesity.

"For this age group parents are obviously a really critical group to be involved and at the moment environmental changes like food taxation and so on are simply not in the realms of possibility."

At the moment the health checks are showing around 15 per cent of Canterbury preschoolers are obese.