Social Development Minister Paula Bennett has spoken to families commissioner Christine Rankin about an interview she gave criticising the so-called anti-smacking 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.

United Future leader Peter Dunne and Green MP Sue Bradford have accused Ms Rankin of campaigning and called on her to step down.

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.

Ms Bennett today said she had reiterated to Ms Rankin Mr Key's position that she should not campaign against the law.

"She certainly been made very clear that we don't expect her to be actively campaigning on the no vote," she told reporters.

Ms Bennett said the interview was Ms Rankin's personal opinion.

"I think it's pretty clear that Christine Rankin was speaking as Christine Rankin and not as families commissioner and in that case I thought it was quite acceptable."

Ms Rankin was entitled to her own opinion, she said.

"I think that when she's speaking as a families commissioner then she is with them on what they are doing but I think that she's entitled to her view as long as she's not actively campaigning against the commission, and what they believe, she is allowed to put a view forward."

Asked what actively campaigning was, Ms Bennett said Ms Rankin should follow the commission's rules and policies.

Prime Minister John Key earlier stopped short of criticising Ms 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.

"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."

But Peter Dunne, who was responsible for the Families Commission being set up as part of a post-2002 election deal with Labour, disagreed.

"She's certainly taking a very clear position... She's taking that in opposition to the Families Commission, to which she has been appointed," he told NZPA.

"If she wants to take a view campaigning for that referendum as she's perfectly entitled to, then she stands down."

If she did not, then Mr Key should act, Mr Dunne said.

"I would have thought that the Prime Minister's view was very clear previously about what his expectations were and I think he now needs to honour those....He's got to live up to the statements he made previously."

Green MP Sue Bradford agreed.

"It's (media interviews are) certainly part of what you do when you are campaigning," she told NZPA.

"It's certainly supporting the vote no campaign."

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 Bradford said the initial listing of Ms Rankin to appear at the no campaign launch was "suspicious".

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.