A controversial psychologist has been criticised for a new report that says New Zealand mothers could be harming their children by sending them to daycare.

The report Who Cares? Mothers, Daycare and Child Wellbeing in New Zealand - commissioned by Family First New Zealand and prepared by British psychologist Aric Sigman - looks at the potential impacts of separating a child from their parent in the first few years of life.

The 30-page report, which drew on previously published research from around the world, said attending daycare, and the subsequent separation from parents, was a significant source of stress for many children.

It said 70 to 80 per cent of children in centre-based daycare had increasing levels of the stress hormone cortisol, which could be harmful to the child's immune system.

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The author has been criticised in the past for ignoring inconvenient evidence, cherry-picking research and relying on outdated studies.

Family First national director Bob McCoskrie said the report was compiled to put a new set of facts on the table.

"[In the past] it's all been about getting mothers back to work and the benefits of daycare in terms of outcomes, but there's been no corresponding weight given to some of the research that's coming through on the effects of long periods of childcare," he said.

"The industry talks about outcomes and school readiness but it doesn't talk anything about what the kids actually go through while they're at childcare."

Carolyn Savage, president of the Federation of Business and Professional Women, said the report focussed on the harms to children but ignored the educational and social benefits of early childhood education.

It also unfairly singled out women because daycare was a decision both parents had to make.

"I think that's one of the worst things about it ... I think it's a little bit sexist, because it hasn't actually noted what fathers could be bringing to the table."

Women were already judged for deciding to send their children to daycare, Ms Savage said.

"This report says you shouldn't be sending your children to daycare and this is the impact you're going to have. But there's already a number of women out there that suffer guilt for doing so."

Ms Savage said taking time off to support a child was not an option all families could afford.

She knew of one woman who had to put her child in daycare six weeks after giving birth, because she and her husband could not have survived without two incomes.

Another had to put a third of her income towards childcare so she could continue to earn money for the family.

Ms Savage said women who took time out to raise their children often took backwards steps in their careers.

"One of the hardest decisions for women when they put their children into daycare is, are they impacting on their children, but also what's going to happen to their professional careers?"

Mr McCoskrie said the report started to look at what mothers actually wanted.

"We want fulltime mothers to be recognised and valued, we want families to be given real choice.

"At the moment some families are choosing to have both parents working, not because they want to and not because they think it is in their child's best interest but because they've got no choice."

He thought the report would be received "like a lead balloon" because it went against the ideology that has been underpinning the push to get children into daycare.

The report's author, Dr Sigman - a Fellow of the Society of Biology and an Associate Fellow of the British Psychological Society - has spoken out in favour of smacking and is a vocal opponent of parents spoiling their children.

His latest work to come under fire was a paper published in the journal Biologist that found sending children to daycare could harm development and have an impact on their future health - the same conclusions as the Family First paper.

Prominent debunker Ben Goldacre, author of the Bad Science column in Britiain's Guardian newspaper noted the Biologist paper was not a systematic review or reasonable summary of the literature, and concluded it was "deliberately incomplete".

In a column defending the research, Dr Sigman responded that claim was "entirely untrue".

Mr McCoskrie said the Family First report drew on about 100 other researchers and their findings and was not Dr Sigman's opinion.

"I think if you looked at the report you would find it's very well balanced, he's been attacked simply because he's daring to go where no one else has tried.

"I think Aric Sigman is just joining the chorus, we've commissioned the report because it needs to be heard."