The use of physical punishment by parents against children has declined, but remains common, a University of Otago study found.
Children's Commissioner Judge Andrew Becroft said the report's findings were encouraging and indicative of generational change, but there was still work to do to stop physical punishment by parents.
The longitudinal Christchurch Health and Development Study started in 1977 and examined 763 parents, both before and after the law changed in 2007 and the anti-smacking legislation that prohibits physical punishment of children was introduced.
The findings show a clear downward trend in the rates of what the report describes as minor assaults, such as smacking on the hand and bottom, and more severe physical punishments, including hitting with a fist.
Minor assaults almost halved from 77 per cent of parents in 2002 to 42 per cent in 2017. Severe assaults reduced by two thirds over the same period, from 12 per cent to 4 per cent.
The most common physical punishments were smacking on the bottom and slapping on the hand, arm or leg.
Parents more likely to use physical punishment tended to be younger, caring for more children and in a violent partner relationship.
The study found that a history of personal mental health problems combined with socioeconomic disadvantage was also influential.
Judge Becroft said while the significant decrease in the use of force was encouraging, that had to be set against the fact it was still used, which was concerning.
"We want much better than that. Our starting point has to be that all children deserve to grow up in a safe and secure family environment. We know that violence harms. There are better ways to bring up children and to ensure discipline and boundaries than hitting."
Co-author Dr Geraldine McLeod, a senior research fellow in the Department of Psychological Medicine, Christchurch, said the rate of physical punishment against children was higher when parents were younger, and then decreased with age.
She said because of the way the study was designed, it couldn't pinpoint how much the rates reduced because of the law change.
Clinical psychologist Dr Melanie Woodfield, from the University of Auckland, told Newstalk ZB many parents experienced a fleeting thought or urge to strike their child and that didn't make them a bad parent.
"For most, a whole lot of things kick in to inhibit their urge to strike their child, to help them regulate their emotions, to reason and think through consequences.
"These processes of inhibition, emotional regulation, executive functioning, they happen in a millisecond.
"For some parents, those processes that are designed
to inhibit that, that might be compromised in some way.
"It might be that that parent is chronically stressed or overwhelmed or isolated or unsupported - or they themselves might have pre-existing difficulties with regulating their emotion."