A national referendum is re-igniting debate on the anti-smacking law two years on, but confusion surrounds the position the two main political parties will take.

Prime Minister John Key helped to negotiate the law and says it is working.

But he says it would be very difficult to interpret anything from the referendum - which he expects to have a low return - even if there were a large No vote.

Labour leader Phil Goff, whose party staunchly supported the law under Helen Clark's leadership, says the law is working. But he says he will probably abstain because the question in the referendum is confusing.

But Green MP Sue Bradford says that Mr Key and Mr Goff should be supporting a Yes vote if they believe the law is working.

She said Mr Goff's talk of abstaining was "really copping out of the issue".

Labour had been a staunch supporter of her bill.

"It seems such a pity to sell that out by taking such a cop-out position now."

Ms Bradford, the sponsor of the original bill, said last night she would like to talk to the Labour Party caucus to find out if leaning towards abstaining on the vote was the official party position.

The offer extended to National and Mr Key, who helped to reach a compromise on Ms Bradford's original bill and enabled it to pass in Parliament by 113 votes to 8.

Postal voting on the $9 million referendum begins on July 31 and ends on August 21.

Citizen-initiated referendums are not binding and Mr Key has said he would change the law only if it was not working.

"I am satisfied that the law is working. I am satisfied that the police have clear riding instructions from Parliament."

He said that if a good parent was criminalised for lightly smacking a child for the purposes of correction "I believe that would be wrong" but often there was a lot more to cases than what surfaced in public.

Asked what he would do if the vote was strongly No, he said it was a difficult question and could be interpreted several ways.

"The problem is it's a bit like a byelection, isn't it. What we saw on the weekend was that not many people turned out and there is every risk for a postal vote that there will be a very low turnout as well."

Mr Goff said the question was badly worded. "The question implies that if you vote 'yes' that you are in favour of criminal sanctions being taken against reasonable parents - nobody believes that."

He said the question should be,"Is the law working satisfactorily?"

Yes Vote Coalition spokeswoman Deborah Morris-Travers said her group had worked hard to communicate with the political parties on their position.

"But the thing we will be investing most energy in will be better informing the public about how the law works and what it means for families... to cut through the nonsense."

The referendum was approved after Sheryl Savill, who worked for the Christian group Focus on the Family, got the required 390,000 signatures last year.

Family First national director Bob McCoskrie said Mr Key was undermining the process by suggesting that while he would listen to the public, any law change would be subject to what he thought.

Mr McCoskrie also criticised Mr Key for saying he expected Families Commissioner Christine Rankin - who opposes the law - would not campaign on the issue.

Citizens initiated referendum question:

* Should a smack as part of good parental correction be a criminal offence in New Zealand?

* Vote Yes if you think the present law is working, says the Yes Vote Coalition including Plunket and Barnardos.

* Vote No say opponents of the present law such as Family First.

* Postal voting starts on July 31.