A victim support group is urging the Government to ignore proposals to include 17-year-olds in the youth justice system.
Social Development Minister Anne Tolley agreed in April that Cabinet would investigate raising the age.
An open letter calling for the change was released on Monday. It was signed by Children's Commissioner and former principal Youth Court judge Andrew Becroft, the Greens, the Salvation Army, Community Law Centres o Aotearoa, Save the Children, Youthline, UNICEF, several university law lecturers and others.
They say New Zealand is in breach of its international obligations by treating 17-year-olds as adults in the justice system.
"If you do not include 17-year-olds in the youth justice system you are failing to take action to break the crime cycle for these children," the letter reads.
"Children already associated with police and justice systems need very specific attention to maximise the chance of them thriving."
But Sensible Sentencing Trust president Garth McVicar says 17-year-olds are old enough to face adult justice.
"Even in the adult court an offender has around on average 10 offences before they go to prison. To say a young offender is going to prison is absolutely a nonsense. There are all sorts of alternative actions to give people a chance."
Mr McVicar says changes need to be implemented to reduce youth offending, rather than to change how young criminals are punished.
But Mr Becroft says Mr McVicar is promoting a "popular misconception" that young people facing the Youth Court would be let off more serious crimes.
"The Youth Court still has open to it very serious sentences where young people can be convicted and transferred to the District Court for sentence," he told RNZ.
"So if public safety is the concern, you can almost guarantee that every 17-year-old who goes to prison now would still go to prison."
Prime Minister John Key backed that up, telling Paul Henry on Tuesday serious crimes would still be dealt with by criminal courts.
He says there is "logic" in the suggestion to raise the age, but won't say whether he backs the idea as it's yet to be looked at by Cabinet. If the change goes ahead, he hopes to have it in place by the end of the year.
The Government is set to answer questions from the United Nations Committee on the Rights of the Child in Geneva on Friday about New Zealand's level of compliance.
Last week Queensland became the last Australian state to raise the youth justice age to 18.
The previous Labour Government was making moves towards raising the age, but National cancelled the plans, the New Zealand Herald reported in 2012.
In 2010 the Government made 12- and 13-year-olds eligible for Youth Court and introduced boot camps.