Children's Commissioner Cindy Kiro has backed the concerns of Principal Youth Court Judge Andrew Becroft over legislation that would lower the age of criminal responsibility.
Dr Kiro has told a committee of MPs that imprisoning children will not help rehabilitation.
Judge Becroft appeared before the committee on Wednesday to make a submission on a bill that would effectively abolish the Youth Court and end family group conferences.
New Zealand First MP Ron Mark's Young Offenders (Serious Crimes) Bill would lower the age of criminal responsibility to 10 - although he has agreed to amend that to 12.
The legislation has been referred to Parliament's law and order select committee for scrutiny after Labour supported its first reading. It agreed to do that under its confidence and money supply deal with NZ First.
Judge Becroft said if MPs wanted to improve the system they should better resource police youth workers, improve training and keep children in school.
Mr Mark's "abysmally drafted" bill was not the answer. "It would effectively end our youth justice system," he said.
Under the bill, only 18 charges - for offences like stone throwing that seldom ended up in court anyway - would remain in the Youth Court jurisdiction.
All other charges would have to go to the adult court and there would be no discretion. Imprisonment would be an option for almost all charges. "It [the Youth Court] would be gutted effectively of its jurisdiction."
Judge Becroft said under present law, children could be tried in adult courts - 10 to 13-year-olds were dealt with in the Family Court and those aged 14-16 in the Youth Court - but serious charges, including sex attacks and violence, went to district or high courts.
The Youth Court could use its discretion to hear cases, except murder or manslaughter, but even then could send a youth back to an adult court for sentencing.
"There is a means now to convict and transfer all young offenders to the district court for sentence," Judge Becroft said.
"It may be that the bill itself and the framers of the bill fail to understand that point."
About 60 youths, usually males, were jailed each year, a figure that had been stable for about a decade.
He produced statistics showing youth offending rates were fairly steady from year to year. For 10 to 13-year-olds, the rates of offending were reducing or stable. "Where is the statistical evidence to suggest offending by 10 to 13-year-olds is spiralling out of control?"
Judge Becroft said the emphasis was to divert youth out of court systems and about 80 per cent of cases were dealt with in the community. " ... we know from the research that most of that 80 per cent diverted away from the system don't reoffend."
Judge Becroft said a large group of youth stopped offending once they were caught while a small group of up to 3000 persistent offenders, usually boys, who were "unexploded human time bombs", were likely to become adult offenders.
Early intervention was the best answer but no one internationally had done any better than New Zealand in reducing reoffending - 55 per cent who faced family group conferences either did not reoffend or did at a significantly reduced level.
Mr Mark hit back at the criticism, saying violent crime was escalating and the judge had focused on offenders not victims.
Judge Becroft said violent crime had increased but that was across all age groups. He hoped the debate would result in better resourcing and standards in the system.
A lack of data because of different methods in Child, Youth and Family and police meant a young person could not be tracked across the system and it "defied belief" there was no unified data set.
Dr Kiro backed the judge, saying imprisoning children would not help rehabilitation and breached the United Nations Convention on the Rights of the Child.
Mr Mark told Radio New Zealand yesterday that he was open to making changes to the legislation so it better met his intentions.
He said his bill could be changed to widen the discretion of the Youth Court to send a wider range of cases to the district court rather than have them automatically covered.