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?"