The chair of the United Nations Committee on the Rights of the Child said the Northern Territory had contravened Australia's international treaty obligations by lowering the age of criminal responsibility from 12 to 10 years old.
Speaking exclusively to ABC News, committee chair Ann Skelton said she was writing to Prime Minister Anthony Albanese to inform him of Australia's treaty obligations as a signatory to the United Nations Convention of the Rights of the Child.
"The treaty talks about the fact that the minimum age should not be set at too low an age," she said.
"Over the last 35 years the committee has interpreted that consistently to say that states must set an age and progressively increase it.
"What is particularly concerning is that once a state has committed itself and has actually set a minimum age, this idea of going back is, I would say, in contravention of the Convention."
The Northern Territory passed new laws to lower its criminal age late on Thursday evening, delivering on a key election promise of the new conservative Country Liberal Party government within its first parliament sittings.
The NT government has become the first in the country to lower its age of criminal responsibility to begin jailing children as young as 10, two years after the territory became the first Australian jurisdiction to raise the age.
The jurisdiction has by far Australia's highest rates of children in detention according to data from the Australian Institute of Health and Welfare.
A recent NT Children's Commissioner report found 94 per cent of imprisoned children were Indigenous.
In a statement to the ABC, Chief Minister Lia Finocchiaro said the laws were not designed to send 10 year olds to jail.
"By lowering the age of criminal responsibility, we can intervene earlier in a young person's life and provide them the support they need to turn a new page, and start a better journey of skills training," she said.
However her government voted against an amendment proposed by Yol?u elder and independent politician Yi?iya Guyula that 10 to 11 year olds would not be sentenced to a juvenile detention centre.
"Territorians overwhelmingly voted for this change in the election seven weeks ago," Ms Finocchiaro said.
"The status quo simply can't continue. Territorians want to feel safe again and our troubled youths need earlier intervention."
The UN has recommended 14 as the minimum age of criminal responsibility worldwide, and 31 member states have previously urged Australia to meet that standard.
Currently, the minimum age of criminal responsibility in every Australian state and territory is 10, except for the ACT, which last year raised its age of criminal responsibility to 12.
The ACT, Tasmania and Victoria have all previously committed to raising the age of criminal responsibility to 14 but Victoria has walked back that promise and is now planning to raise it to 12.
'It's a bloody national crying shame'
National Children's Commissioner Anne Hollonds requested a meeting with Ms Finocchiaro ahead of the introduction of the new law.
After the law passed she called on Mr Albanese to make child welfare and justice a national cabinet priority.
"The fact that this law has been passed in the Northern Territory last night is evidence of the failure of all the other systems that should have helped these children and their families much, much earlier," she said.
"It's deeply disappointing that the criminal justice system is seen as the only solution to addressing the problems they have."
Co-founder of the North Australian Aboriginal Justice Agency Natalie Hunter said the passing of the laws marked a "national day of sorrow for our people."
The Nyikina woman said she was shocked the government had not passed the change without establishing rehabilitation and diversion programs, including its two promised "boot camps".
"How can you make a law when you don't put in the support base first?" Ms Hunter said.
"We don't even have the support base now with what's going on at Don Dale Youth Detention Centre.
"I worked in the child protection system for 20 years in the Northern Territory — it's a bloody national crying shame."
Ms Hunter said she believed the changes were racist and would not make the community safer.
"We know that's going to target Aboriginal children," she said.
"Jailing kids makes crime worse — it doesn't make things better … [Lia Finocchiaro] is just going on the vote to keep her in power, and that's really sad."
The NT's Catholic bishop in Darwin, Charles Gauci said he had heard concerns from community members including Aboriginal elders about the "rapid pace of the new legislation".
He urged the government to proceed "with wisdom and proper consultation".
"Legislation alone isn't the full solution, we must also focus on long-term strategies that truly benefit all Territorians," he said.
The ABC contacted the prime minister's office for comment multiple times, but did not get a response before deadline.
