Groundbreaking New Zealand research has refuted thousands of international studies which claim that smacking children makes them more likely to become aggressive and antisocial.

Children who are smacked lightly with an open hand on the bottom, hand or leg do much the same in later life as those who are not smacked, found the Dunedin multidisciplinary health and development study, which has tracked 1000 children since they were born in the city in 1972-73.

The finding, based on interviews in the past two years when the children were 32-year-olds, will be published this year.

An earlier part of the study published in the NZ Medical Journal in January, found that 80 per cent of the sample had been physically punished at home during childhood.

Twenty-nine per cent of the whole sample had only ever been smacked. A further 45 per cent had been hit with an object such as a strap or wooden spoon, and 6 per cent had suffered "extreme physical punishment" that left cuts, lasting bruises or welts or involved out-of-control hitting, choking, being thrown or sexually violated.

Numerous overseas studies have shown that children who are physically punished are more likely to be aggressive and antisocial, have poor parent-child relationships and develop mental illnesses.

But the lead author of the physical punishment part of the Dunedin study, psychologist Jane Millichamp, said the project appeared to be the first long-term study in the world to separate out those who had merely been smacked with an open hand.

Preliminary analysis showed that those who were merely smacked had "similar or even slightly better outcomes" than those who were not smacked in terms of aggression, substance abuse, adult convictions and school achievement.

"Study members in the 'smacking only' category of punishment appeared to be particularly high-functioning and achieving members of society," she said.

"I have looked at just about every study I can lay my hands on, and there are thousands, and I have not found any evidence that an occasional mild smack with an open hand on the clothed behind or the leg or hand is harmful or instils violence in kids," she said.

"I know that is not a popular thing to say, but it is certainly the case.

"The more honest researchers have said, let's be honest, we all wish we could say it's all very clear and that no parent should ever lift a finger on a child - although I think that is totally unrealistic as a single parent myself - but the big problem is that a lot of the studies have lumped a whole lot of forms of physical punishment together."

Dr Millichamp said the Dunedin study so far found no evidence of the "slippery slope" theory - that parents who started off smacking often progressed to abusive punishments.

"We couldn't find any," she said.

The findings undermine Green MP Sue Bradford's bill to repeal section 59 of the Crimes Act, which allows parents to use "reasonable force" to discipline children.

Dr Millichamp said there was no doubt that abusive punishments had long-lasting negative consequences, but the research did not support banning mild smacks.

"It's unethical to make out that there is a lot of evidence that mild smacking is harmful," she said.

She and colleague Judy Martin have made a written submission to Parliament suggesting that section 59 should be retained but amended to allow smacking with an open hand, but not hitting with a closed fist or certain objects.