Obesity spanned all ethnicities, genders and levels of deprivation.
Dr Tuohy said obesity was particularly worrying in children because it was associated with a wide range of health issues.
"The World Health Organisation recommends schools create healthy food environments so introducing a water-only policy is a great first step for schools," he said.
"This announcement also supports the work we are doing with the Ministry of Education and other agencies to implement the childhood obesity plan."
One school working towards a water-only policy is John Paul College in Rotorua. Principal Patrick Walsh said the school had built two filtered water stations, which were proving popular.
"I've found at secondary school, many of the students are actually quite health-conscious."
The school still offers sports drinks and flavoured milk in its tuck shop, but Mr Walsh said he was not fooled by the perception these drinks were healthy.
"They may not have as much sugar in them as fizzy drinks do but they are not as healthy as they are marketed to be."
- Staff reporter