Feeding hungry schoolchildren does nothing to boost their learning, a new report shows.

The findings have surprised experts in a week when campaigning to introduce free food at schools to combat child poverty put pressure on the Government.

The only "significant positive effect" was that children felt less hungry, the study into free school breakfasts found.

Head of the study, Associate Professor Cliona Ni Murchu, said there were indications that attendance at school was also likely to improve but in reading, writing and maths there was no noticeable improvement.

Researchers at Auckland University's School of Population Health studied 423 children at decile one to four schools in Auckland, Waikato and Wellington for the 2010 school year.

They were given a free daily breakfast - Weet-Bix, bread with honey, jam or Marmite, and Milo - by either the Red Cross or a private sector provider.

Despite the findings going against the assumption that well-fed children concentrate better and therefore do better at school, the report has not deterred the advocates of free food at schools.

"I'm not a researcher but I have been in the game for 36 years and I would support taking an educational role around diet," said John Coulam, president of the Waikato Principals Association.

"A positive of the breakfast programme is it educates children on what constitutes a healthy breakfast."

He said children often replaced an unhealthy or insufficient breakfast at home with a balanced nutritional one at school.

Ensuring children were well fed was also part of the cure to New Zealand's greater poverty problem, said Jacinda Ardern, Labour's spokeswoman for social development and children.

"We know poverty has an effect on education achievement. The food in schools programme is one way government and schools can fill a gap that exists because we don't think that we can stand by while children go hungry," she said.

An expert advisory group working for the children's commissioner this year estimated 270,000 New Zealand children - roughly one in four - live below "recognised poverty thresholds".

Labour has launched a $10 million policy to provide free food to 650 of the country's lowest decile primary and intermediate schools.

Prime Minister John Key rejected the idea, saying free fruit was already provided in "vast bulk" at low-decile schools and many had existing breakfast programmes.

Ni Murchu said there was a chance her study did not capture the children who most needed the breakfasts.

"There's always a risk that the kinds of people who participate are not the higher needs group." This was because her study participants had to get parental consent and fill in a lengthy questionnaire - a process that may have alienated the high-needs families.

Broadcaster John Campbell has led the charge for free school lunches this month and he drew a distinction between breakfasts, which the study focused on, and lunch.

Lunch was more likely to have an impact because it came at "a time when kids have done three or four hours and they go into the afternoon and are really lacking energy - more likely to drift off, be inattentive and get things wrong".

Lacking lunch also created stigma for a child, he said, continuing his call for a free $3 school lunch for children who needed it.

"Whatever we can do it is worth a shot. We will all do better if these children do better."

He said one positive was that the issue was on the main political agenda now.

Jonathan Boston, professor of public policy at Victoria University and co-chair of an expert advisory group of solutions to child poverty, said the research felt "very counterintuitive" but still warranted attention.

"If you came to the conclusion that providing food for disadvantaged children coming to school without will only benefit hunger then you have to say hunger matters - surely we don't want children sitting in school being hungry. Surely relieving their hunger is adequate justification."