The number of children in detention in New South Wales has jumped by more than 30 per cent, sparking warnings from the state's peak Indigenous legal body that the government is ignoring evidence-based policy.

Figures released on Thursday by the NSW Bureau of Crime Statistics and Research (BOCSAR) show that as of June 30, there were 234 young people in custody — 225 boys and 9 girls. Of those, 140 (59.8 per cent) are Indigenous.

More than 70 per cent of these young people are on remand, despite the 1991 Royal Commission into Aboriginal Deaths in Custody recommending detention be used only as a last resort.

The proportion of Aboriginal adults in custody has also reached 33 per cent — "another shameful new record for NSW," according to the Aboriginal Legal Service (ALS) NSW/ACT.

ALS chief executive and National Aboriginal and Torres Strait Islander Legal Services (NATSILS) chair, Karly Warner, said the numbers reflect the state government's "continued disregard" for Closing the Gap commitments.

"Despite committing to Closing the Gap in the over-imprisonment of Aboriginal people, the NSW Government continues to enact laws and policies that drive up mass incarceration," she said.

"Last year, the Premier introduced unprecedented child bail laws which have been condemned by legal and community experts for making it harder for children to access bail than adults charged with the same offences."

The laws, which were strengthened this year, limit access to bail for children aged 14 and over accused of break-and-enter and motor vehicle theft while already on bail for similar offences, and place further restrictions on bail for accused domestic violence offenders.

Ms Warner said the consequences were predictable.

"The government knows that 85 per cent of children refused bail under the new laws are Aboriginal, yet decided to extend them anyway for another three years," she said.

"There is no evidence anywhere that tougher bail laws reduce crime. Our children are being sacrificed at the altar of the dangerous and regressive law-and-order politics sweeping the continent."

BOCSAR executive director Jackie Fitzgerald said the reversal of a decade-long decline in youth detention numbers is particularly troubling for Aboriginal communities.

"This increase is particularly concerning for Aboriginal young people, who were already significantly overrepresented in the justice system," she said.

"It also places NSW at odds with our Closing the Gap target to reduce the number of Aboriginal young people in detention."

Premier Chris Minns defended the government's position, saying the changes were introduced because "the system wasn't working".

"Repeat offenders were being arrested, given bail, and then the charges were either being dropped or dismissed," he told reporters.

"Our sole objective is to stop reoffending and to get a young person who's lost their way back on track."

State Greens MP Sue Higginson said the data shows the bail laws "disproportionately target First Nations young people and fuel a surge in youth imprisonment," calling it a "devastating and predictable outcome".

"We know that locking up kids is the most criminogenic thing we can do to them, it increases the likelihood of reoffending, entrenches disadvantage and drives higher crime rates," she said.

Justice and Equity Centre chief executive, Jonathon Hunyor, said the state's approach was "letting down children and communities".

"We are locking up more and more children on remand who aren't given a jail sentence," he said.

"This harms these children, makes crime more likely and doesn't keep our communities safe. We need to stop investing in failure and start investing in our communities."

Ms Warner said the situation in NSW reflects a "national crisis" and urged the Prime Minister to take action at the highest level.

She called on Anthony Albanese to put "justice for Aboriginal and Torres Strait Islander people on the agenda at National Cabinet" and lead "urgent, coordinated reform" based on the advice of the National Justice Policy Partnership.

The ALS continues to push for alternatives to prison and police-led responses, favouring "therapeutic, community-led" approaches.

"Locking up children does not prevent crime. All it does is destroy lives and make communities more dangerous," Ms Warner said.