DARWIN, July 12 - As of Monday, 18 out of 33 detainees were kept in the HSU, which used to be a maximum security section of an old adult jail.
That's risen by five in just over a week.
The NT has the highest rate of youngsters behind bars, 96 per cent of which are Aboriginal.
In a submission to the juvenile detention royal commission, Human Rights Law Centre Senior Lawyer Shahleena Musk said a youth justice system modelled on adult prisons is a "recipe for failure".
"Most of the children in the youth justice system have suffered significant disadvantage, trauma and health problems," she said.
Four of the 18 boys only had medium or low security classifications, but Territory Families says they may still be kept in the HSU if they have an escape history or are at risk of harming other inmates.
Ten of those in the HSU are on remand, while eight have already been sentenced.
For years these cells had little natural light or airflow, but Territory Families says recent upgrades have fitted every room with "improved windows, air-conditioning and individual room water coolers".
The department has also stressed that kids in the HSU are not kept in solitary confinement.
Boys being tear-gassed, spithooded and shackled to restraint chairs in NT youth prisons sparked the inquiry last year and prompted Labor to inject $18.2 million to overhaul the broken system.
The Gunner government has committed to building new youth jails in Darwin and Alice Springs but is waiting for the inquiry's recommendations, due September 30, before deciding on a final plan.
No money was allocated for the facilities in this year's budget, but Labor has pledged $15.3 million for bail support accommodation and safe houses to help juveniles awaiting trial.
The Territory's only remote rehabilitation boot camp on Tuesday closed its doors, blaming inaction by the government on security and safety issues leading to breakouts.
The HRLC calls for more investment in indigenous-led alternatives to detention, and said any reforms must be evidence-based instead of just "making it up as they go along".
"The NT government must shift its priorities away from shoring up failed youth detention centres that warehouse children towards proven community-based options like education, training and drug and alcohol programs that help kids get their lives back on track," Musk said.
DON DALE INMATES AS OF JULY 10:
- Total number in detention: 33 (3 girls and 30 boys)
- HSU: 18 boys (1 low security classification, 3 medium, 14 high)
- 10 on remand, 8 sentenced.
(Source: Territory Families)