Criminal assaults by children of primary school age soared last year, and police were called to more than one case a week.
Crime statistics show the number of children under 9 apprehended for assaults last year was 64, almost double the 33 recorded in 2009.
Forty-four of the 64 were boys.
Four children were apprehended for serious assaults causing injury, and 52 for common assault.
There were also more assaults in the 10-13 age group - there were 827 apprehensions last year, against 770 in 2009. Again, the majority of offenders were boys.
In New Zealand, the age of criminal responsibility is 10 and any offender under 16 not charged with a crime can be referred to Youth Aid or face police warnings or family group conferences.
Psychologist Sara Chatwin said it was clear that exposure to violent television programmes led to more aggressive and violent behaviour in children.
"More and more, we're seeing this kind of stuff on the screens and children and young people have more access to it. A large percentage of this behaviour is copycat stuff."
Ms Chatwin said another reason children were more readily committing assaults was a breakdown in parental control.
"Families under stress, parents under stress, parents working hard to feed their families, not having enough time to put into discipline, and kids being left to their own devices."
Ms Chatwin said recent cases of bullying in which attacks had been filmed and put on the internet for all to see proved violence was becoming more acceptable among young people.
"We're seeing the extent of the violence ... in the schoolyard. It's hideous and unbelievable."
Criminologist Greg Newbold believed violence was becoming glorified, especially among young girls.
"It's a really good example of the kind of mentality our young girls are growing up with. They see movies and video games of 'kick ass' women and play it out in reality."
Dr Newbold also blamed the violence of the mass media.
"In the past five years, technology has increased exponentially. Kids get more and more realistic imagery in front of them and start living that imagery out.
"When I was a kid, we had comic books with really watered-down and soft fights, and cowboy movies. Nowadays, it's really graphic."
Dr Newbold said children growing up in stable homes were less likely to be sucked into thinking the violence they saw in the mass media was okay.
"But someone brought up in an unstable environment and exposed to violence can take on these messages and want to play them out."
He said as long as children continued to be exposed to violence in their lives, the assaults would carry on.
"I certainly think that the accessibility of those video games to younger and younger age groups will inevitably result in an increase in violence."
Ms Chatwin said there was no easy solution.
"There is no quick fix," she said. "It's the biggest question we as New Zealanders have to ask ourselves - are we doing enough to look after our kids? Are they enough of a focus?
"The answer may be no. At every level there is some work that needs to be done, from parents and families, cultural groups, educators and politicians.
"It is not an easy problem to fix, but there is a need for action."
* 64 children under 9 were apprehended for assault in 2010 compared with 33 in 2009
* 44 boys under 9 assaulted someone last year
* Four children under 9 committed serious assaults causing injury
* 52 children under 9 were apprehended for common assault
* 827 children aged 10-13 assaulted someone last year compared to 770 in 2009.