A top government adviser says the number of children dealt with by police for assault is "the tip of a very big iceberg" and more than 40,000 others could follow in their violent footsteps.

Dr John Langley said up to 47,000 children and young people within the compulsory school system could be described as having conduct or significant behaviour problems.

"It's the single biggest social problem New Zealand has, without a doubt," he said.

"Not all of those kids will necessarily be at the extreme end, but a good portion of them will be."

Dr Langley is part of an advisory board set up by the Ministry of Social Development to prepare a series of reports on children with conduct problems including antisocial, aggressive, dishonest, disruptive and violent behaviour.

Yesterday, the Herald revealed that the number of children younger than 9 dealt with by the police for assaulting others almost doubled last year from the previous year.

Dr Langley said the advisory board's reports and suggestions would be used to develop a comprehensive plan for dealing with such children.

"I don't think the extent of this problem until recently had ever been recognised. It impacts on the lives of hundreds of thousands of people one way or another.

"What I think has been recognised in the last two to three years is that if we want to respond to this properly, we have to do it in a very systematic and thorough way."

Dr Langley said there were a number of reasons children developed conduct disorders.

"In the majority of cases, the behaviour is learned. And it's learned because of the environmental conditions the child is in.

"Many of the kids are brought up in places where there are very few rules, or at least consistent expectations on what they should do.

"Prosocial behaviour - the kind of behaviour most kids learn - isn't learned in those environments. These kids learn other things and what they learn are consistent and rational for them."

He said the behaviour scale ranged from whining and "pestering" to violent and aggressive behaviour like lighting fires and using weapons.

He said there was no "silver bullet" or quick-fix solution but the younger the children were when intervention happened, the better.

"What we do know is that if we can intervene early ... we have something like an 80 per cent chance of sorting the behaviour out.

"Once the kids reach the age of about 10 or 11 it drops to about 50 per cent and once they hit the teenage years, while it's possible, it becomes vastly more difficult ... Often then it becomes a chronic lifelong problem."

Dr Langley said if the right approach was taken, the number of children with conduct problems would rapidly decrease.

"In two or three generations we could be looking at a vastly different New Zealand. But unless we're prepared to put some work into the problem, it's the same old story - if you keep doing what you're doing, you'll keep getting what you're getting."