A 5-year-old and two 6-year-old boys were apprehended by police for sexual assault last year.

They were among 716 children aged under nine who were reprimanded in the 2008/09 financial year for offences from unlawfully taking a bicycle and possession of cannabis to threatening to kill and assault with a weapon.

Although the numbers are slightly up on the previous year, they are about half those of a decade ago.

Police say early intervention has helped, but the national manager of police youth services, Superintendent Bill Harrison, said he was shocked and dismayed when hearing about young children committing offences.

The youngest children apprehended in 2008/09 included a 4-year-old girl, two 5-year-old girls and five 5-year-old boys.

They were reprimanded for dishonesty offences such as shoplifting, burglary or theft of property valued under $500.

The 5-year-old and two 6-year-olds who committed sexual assaults were among 12 children who committed sexual offences. Police did not give details of the offending, citing privacy issues.

The numbers - 525 boys, 99 girls and 92 whose sex was not recorded - were released to the Herald under the Official Information Act.

An Auckland University psychologist, Associate professor Ian Lambie, said children who committed crimes generally modelled their behaviour on that of their parents.

"High levels of aggression when very young are usually due to the family environment," he said.

"If parents are violent, involved in alcohol or drug abuse or have psychiatric problems their children are more likely to act out."

Of the children apprehended, 333 were warned or cautioned, and 286 were referred to the police's youth aid section.

Mr Harrison said youngsters could be handcuffed and put in the back of a police car, but it was rare.

"There will be occasions when, if someone is violent, aggressive and completely unresponsive to reasonable intervention to try to calm them down, you're not necessarily going to know that they're nine ...

"I've heard of situations where they're being arrested for serious offences and police have determined that the best course of action is to put handcuffs on them to ensure the risk to the police staff and other people is mitigated."

A child arrested and charged for a serious offence could be held in a cell for up to 24 hours but Mr Harrison had not heard of this happening with a child under nine. Older children might be locked up to be taught a lesson.

"They've decided to give them a short, sharp shock, in conjunction with the parents who want to introduce them to what a cell looks like. But that's not encouraged, it's certainly not police policy."

Children could be made to appear before a judge in the Family Court although not standing in a dock but flanked by their caregiver or a youth advocate.

But no person under the age of 10 can be convicted of an offence.

Mr Harrison said police engagement with the community, including having officers in schools, probably explained the decrease in the number of young offenders.

The police youth services manager for the Auckland district, Senior Sergeant Alan Rowland, said police, Child, Youth and Family and school programmes encouraged early intervention with children who had behavioural problems.

"In the last 10 years we have put a lot of effort into building relationships with these groups."

The figures:

Of 716 children aged under 9 apprehended by police:

* 12, including one aged 5 and two aged 6, committed sexual offences.

* 9 assaults with a weapon.

* 4 offences against animals.

* 1 threatening to kill.

* 5 possession of cannabis.

* 34 arsons, including 13 wilful damage to property endangering life.

* 116 wilful damage.

* 156 shoplifting.