The Children's Commissioner has said a bill which would lower the age young offenders can go before adult courts was an over-reaction which could result in more offending.

NZ Zealand First MP Ron Mark drafted a member's bill -- which passed its first reading on Wednesday night -- aimed to lower the age of criminal responsibility from 14 to 12 years.

Mr Mark said the youth justice system was failing, it was too lenient and more needed to be done to deter young people from a life of serious crime.

But Commissioner Cindy Kiro said the Young Offenders (Serious Crimes) Bill could have negative impacts.

"A young person charged with stealing a litre of ice-cream would be dealt with next to adult criminals in the District Court," she said.

"This sort of over-reaction to offending by a young person and their possible placement in a prison-like environment can only lead to an increased likelihood of future offending by these young people and certainly does nothing to rehabilitate or improve outcomes for themselves or society."

Justice Minister Mark Burton earlier said that under the existing law the age of criminal responsibility was 10 and the age of prosecution was 14 years except for murder and manslaughter where the age is 10.

At 14 years, a young person could be prosecuted for the full range of offences and their offending was dealt with in the Youth Court.

Child offenders between the ages of 10-13 were dealt with in the Family Court.

Dr Kiro said the bill reduced the age children went to adult court for a wide range of criminal offences.

"The legislation would mean that if for any offence where the maximum penalty is imprisonment for a term of not less than three months or a fine of not less than $2000 -- i.e. for almost all offences -- the charge would have to be laid in the adult court.

"This removes from the Youth Court jurisdiction over virtually all criminal offences with which a young person or child can be charged."

Dr Kiro would seek an independent legal opinion on the bill and she believed as it stood it could result in breach of international conventions, including the United Nations Convention on the Rights of the Child.

Mr Mark said his bill was a chance to address the shortfalls of the current youth justice system.

"New Zealand First has long been calling for tougher penalties for serious youth offenders and an attack on entry-level crime to stop young offenders from developing into hardened criminals." He said the bill was about early intervention and deterrence.

"Young offenders who commit serious crimes need to be made accountable for their actions."

The bill will be scrutinised by a parliamentary committee after it was supported by Labour, National, United Future, ACT and the Progressive Party for the first reading.