Prime Minister John Key has stopped short of criticising Christine Rankin for her renewed criticism of the anti-smacking law - but he has made it clear he will not tolerate any "active campaigning" by the families commissioner before the referendum on the law.

In an interview published in Investigate magazine, Ms Rankin attacked the anti-smacking law, saying it had traumatised families.

She said it was a parent's right to smack as a form of discipline.

Mr Key has forbidden Ms Rankin to campaign on the anti-smacking law referendum.

She has always opposed the measure, and a trust she heads is campaigning against it.

Neither Mr Key nor Social Development Minister Paula Bennett was told of the interview, but yesterday the PM said he did not believe Ms Rankin had contravened his edict.

"I don't think it's particularly provocative. I made it clear I wouldn't want to see her campaigning on the 'no' vote, and I think she's honoured that, so I'm pretty relaxed about it."

Mr Key conceded that if Ms Rankin did voice a strong view on next month's referendum, it could be seen as defying his instruction.

Ms Rankin's name was initially on the list of speakers for yesterday's opening of the "Vote No" campaign led by Family First lobbyist Bob McCoskrie, but it was withdrawn at the last minute.

Mr McCoskrie said inclusion of her name was a mistake that he had moved to correct.

Ms Rankin did not return Herald phone calls yesterday. But Family and Children Trust spokeswoman Bev Adair said Ms Rankin was the trust's chief executive and would definitely take part in the referendum campaign at some stage. "She's right behind this campaign."

The Families Commission recently re-confirmed its continued support for the law.

Mr Key said he did not believe Ms Rankin was undermining the commission's practice of presenting a collective view on issues.

"I don't think we should be absolutely pedantic about that," he said.

"From time to time she might make the odd comment, but there's a big difference between a broad characterisation of events and actively campaigning - and that would be unacceptable."

The commission's position on the smacking debate had been formed before Ms Rankin's appointment as a commissioner.

"So I don't think anyone is expecting her to have a remarkable turnaround in her point of view."

In Investigate, Ms Rankin said parents who agonised over bringing up their children were now scared of being reported if they were seen saying something to their youngsters or grabbing their arm.

The "power dynamic" was distorted by the law, which had resulted in 5-year-olds coming home from school telling their parents they could not smack them.

She believed the smacking law was a "smokescreen" that diverted attention from child abuse, an area in which she said the law was too weak.

It was appalling that parents convicted of child abuse could go on to have more children and Child, Youth and Family was powerless to do anything other than watch.