Leading public lawyer Mai Chen says New Zealand's 2007 changes to smacking law have criminalised "good parents".
A legal opinion signed by Ms Chen for the lobby group Family First says case law since Section 59 of the Crimes Act was changed have confirmed that is now illegal for parents to use force against a child for the purpose of correction, even if the force is "reasonable".
"Therefore, in our opinion, statements made by politicians to the effect that the new Section 59 does not criminalise 'good parents' for lightly smacking their children are inconsistent with the legal effect of Section 59 and the application of that section in practice," the opinion says.
Ms Chen and her firm Chen Palmer declined to comment on the opinion, but Family First director Bob McCoskrie said the law should be changed again, in line with the Australian state of Victoria, to clarify that "light smacking" should be allowed as long as it did not involve either the use of implements or hitting a child's head or neck.
"We are calling for the decriminalisation of light smacking. Eighty-eight per cent of New Zealanders called for that in a referendum [in 2009]," he said.
Police reviews show that police investigated 143 alleged cases of "smacking" and 435 allegations of minor acts of physical discipline, such as slapping and hitting children, in the first five years of the new law up to June 2012.
Eight cases of smacking and 47 alleged minor acts of physical discipline were prosecuted. Most parents convicted on these charges were sentenced to minor penalties such as supervision or suspended sentences, No one was jailed, and the most severe penalty reported was 200 hours of community service for a father who was drinking at the time, struck his son across the back of the head with an open hand causing him to fall forward and start crying, and then pulled him backwards and told him to go to sleep.
The new Section 59 states in subsection 1 that parents are justified in using force that is "reasonable in the circumstances" against a child to prevent or minimise harm to the child or another person, to stop the child committing a criminal offence, to stop "offensive or disruptive behaviour", or to perform "the normal daily tasks that are incidental to good care and parenting".
But it adds: "Nothing in subsection 1 or in any rule of common law justifies the use of force for the purpose of correction."
Another subsection adds: "To avoid doubt, it is affirmed that the police have the discretion not to prosecute complaints against a parent of a child... where the offence is considered to be so inconsequential that there is no public interest in proceeding with a prosecution."
Chen Palmer found two cases where parents were convicted in lower courts but the Court of Appeal discharged them without conviction on the grounds that the consequences of conviction were "out of proportion to the gravity of offending".
In one case, a mother asked her husband to hit their son with a belt after a sexual incident with a younger female cousin. Both parents lost their jobs because of their convictions, and the Appeal Court found in 2012 that this was "out of all proportion" to the offending, which came after the parents had sought help to cope with the child's attention deficit disorder and had tried several non-physical measures.
In the other case, the Appeal Court last year overturned the conviction of a father who smacked his two sons a total of about a dozen times in two and a half years. It said the consequences of conviction, which included losing custody of his sons and "real consequences for his employment", were "out of all proportion to the gravity of offending".
However, convictions have stood against a father who slapped his 13-year-old daughter on her leg after she was found in a car with her 16-year-old boyfriend, and a grandmother who used her foot to restrain her 3-year-old grandchild who threw a tantrum at a playground.
Chen Palmer commented: "As a result, non-lawyers, including parents and the police, will have difficulty applying Section 59 in practice. Parents will struggle to know whether their actions constitute an offence under Section 59 or not, and in cases of doubt, police will prosecute and leave it up to the Court to determine."
Children's rights lawyer Alison Cleland said the Chen Palmer opinion missed the point that children should have the same rights as adults not to be assaulted.
"What you have is a legal opinion that says yes, you have made it a criminal offence. Of course it has - big deal," she said.
"You have due process, and the Court of Appeal shows that. You have checks and balances there, like you do with anything else."
On the web: familyfirst.org.nz