Drastic cuts are needed to the number of junk food ads, if children's health is to get any better.

A world-first study by Otago and Auckland universities has found New Zealand kids are exposed to 27 junk foods ads every day.

It includes ads on television, in schools, at dairies or on public transport.

Advertising consultant Mike Hutcheson told Rachel Smalley pester power is a tool that's often used, but it's naive to think it's the only problem.

"The real basic problem is that bad food is cheaper than good food. I think that that's a major cause in the obesity epidemic particularly in lower economic areas."

LISTEN ABOVE AS MIKE HUTCHESON SPEAKS WITH RACHEL SMALLEY