Prime Minister John Key says a new review gives parents the go-ahead to lightly smack their children without the fear they will be investigated and prosecuted for doing so.

Mr Key yesterday presented the findings of the review into how the anti-smacking law is being used, ordered after a referendum heightened calls for the legislation to be repealed.

The review panel - clinical psychologist Nigel Latta, Police Commissioner Howard Broad and Ministry of Social Development chief executive Peter Hughes - found police and social workers were acting proportionately and one-off complaints about light smacking were not investigated unless other circumstances were at play.

Mr Key said it showed parents were still able to lightly smack and he personally believed it was already acceptable, despite the strict letter of the law. If that changed, he said, he would change the law, as he had promised.

"Lightly smacking a child will be in the course of parenting for some parents and I think that's acceptable. It is up to individual parents to decide how they're going to parent their children ... Some people will continue to lightly smack their child for correction, some will not. It is up to them to decide."

Dr Latta was given access to all police and social welfare files for the cases the Family First lobby group used in its bid to overturn the law. It had claimed the parents in the cases were inappropriately investigated.

However, Mr Latta said although some seemed at first glance to be an overreaction, after viewing all the background information he believed investigation was justified.

All had aggravating factors, such as a history of domestic violence, that Family First might not have known about because it had no access to the files, he said.

Other features included a child giving a different version from a parent's of the scale of violence - such as a case where a father said he smacked his 13-year-old once on the leg, but she said he hit her on the head with a telephone book and punched her.

Said Mr Latta: "In all the cases, there wasn't once where I thought the only reason they investigated was because of the law change. In every case, when I looked at the police file, it was blindly obvious why they had."

He said one-off complaints about light smacking were not enough to trigger an investigation unless there were other circumstances. Most cases, if not all, would have been investigated under existing abuse provisions, whether or not the smacking law existed.

While there was potential for overreaction, that had always been the case and it was safer for police to err on the side of caution when making decisions about the safety of children.

Family First spokesman Bob McCroskie said the review was a "government-funded sales pitch" that ignored the real concerns about the law. It required too many "compromises, guidelines, helplines, reviews and parental education", he said, and he urged the government to adopt Act MP John Boscawen's private member's bill, which would specifically allow light smacking.

Mr Key said the government would adopt the recommendations in the report, including a helpline for parents facing investigation.