The legislation closes a legal loophole that allowed parents to use "reasonable force" to discipline their child.

However, the new bill allows the police to use discretion over whether a parent should be prosecuted or not.

Supporters say the previous law was too lenient, but critics say the new rules intrude too far into family life.

The bill was passed overwhelmingly, with o­nly seven MPs voting against it.

It amends the New Zealand's Crimes Act, to remove the provision of "reasonable force" for parents in the disciplining of their children.

Critics of the old law said such a provision had helped acquit parents in child abuse cases.

But the changes - first proposed in 2005 - stirred huge concern from conservative groups worried that it would make criminals of parents.

To allay fears, a clause was added giving police the power "not to prosecute complaints.. where the offence is considered so inconsequential there is no public interest in proceeding with a prosecution."

Poor record

The bill's sponsor, Green Party MP Sue Bradford, said the aim of the new law is to stop child-beaters from using "reasonable force" as a legal defence - and not to prevent a parent from smacking a child.

Although police have the power to turn a blind eye to "inconsequential" incidents, opponents of the new law say it does not make clear what that entails.

"The problem with the bill is that while it will allow light smacking, it will do so under circumstances almost impossible to interpret," Winston Peters, of the New Zealand First Party, said.

Smacking is banned in some European countries, but is not in most parts of the world.

New Zealand has a poor record of abuse and neglect of children when compared with other developed countries.

PM Helen Clark has said in the past that she hoped the law would correct that.

"New Zealand has o­n its conscience that our rate of child death and injury from violence, including in the home, is appalling," she said.