A ban on smacking children has been thrust onto the election agenda, with MPs already debating whether a proposed law change would make criminals of parents.

Green MP Sue Bradford last night insisted that her private member's bill unveiled yesterday would protect children and would not see police invading people's homes to arrest a parent who lightly smacked a child.

But opponents said her attempt to change the law would not save any child from brutality.

Ms Bradford's bill will be debated in Parliament for the first time late next month and would repeal the controversial Section 59 of the Crimes Act.

That allows a parent to defend an assault charge by arguing that he or she was using "reasonable force" in physically punishing a child.

The section has been heavily criticised in high-profile cases of child abuse.

Ms Bradford said allowing a parent to use such a defence for beating a child was "barbaric".

Section 59 was cited last month by lawyers for a North Otago woman who was found not guilty of assaulting her son despite admitting she struck him with a horse whip and a bamboo cane.

Other cases include that of a Hamilton man acquitted after using a rubber hose to beat his daughter, which left a raised 15cm-long lump on her back.

That case prompted Prime Minister Helen Clark to declare in 2003 that the benchmark for "reasonable force" was unacceptable and to call for an end to abusers using Section 59 to hide behind.

"I do ask our fellow citizens to reflect on the extent to which that law is shielding and protecting those who are violent," she said at the time.

It is not known how Helen Clark will vote on this bill, but a split within Labour saw it step back from introducing its own anti-smacking legislation before this year's election.

Instead, it pumped almost $11 million into a two-year public education campaign to persuade parents not to physically discipline their children.

MPs will now not be able to avoid the debate, and the vote in the socially conservative Parliament, which may be as soon as July 27, is expected to be close.

When asked last night, most political parties had not yet decided if their MPs would be given a conscience vote, or be required to vote along party lines.

Ms Bradford will need significant support from Labour MPs for the bill to be sent to a select committee for public scrutiny.

She told the Herald the present law on smacking allowed parents to use wood, whips and canes to beat their children.

She denied that the bill would turn parents into criminals.

"I don't believe any policeman would go into someone's home and arrest them for lightly smacking a child."

Justice Minister Phil Goff said he did not oppose the bill going to a select committee, but did not want to make criminals out of parents.

"As a parent, I know the sort of frustrations and the tensions that can build up, and parents, from time to time, will smack their children."

He said Section 59 did not allow the use of unreasonable force, and "child abuse is child abuse".

United Future is expected to oppose the legislation, and MP Murray Smith told TV3 it would make criminals of parents.

National, New Zealand First and Act have yet to discuss the legislation.

The bill cannot pass into law before the election, but if it passes its first reading it can be carried over for a new Parliament to debate.

The law

* Section 59 of the Crimes Act allows a parent or guardian to say they used "reasonable force" in disciplining a child.

* Sue Bradford's bill will repeal that so parents cannot argue the force was justified.