Inmates in the Northern Territory are reportedly being kept in isolation in a Darwin prison for significant periods of time, with the Territory allegedly being investigated by anti-torture watchdogs.

On Wednesday, the NT ombudsman tabled a report in parliament into the usage of prolonged isolation practices - referred to as separate confinement practices (SCP) - in Darwin Correctional Centre (DCC).

In a statement from ombudsman Peter Shoyer, along with children's commissioner and Larrakia woman Shahleena Musk and principal community visitor Jeswynn Yogaratnam, it said the investigation reviewed instances of separate confinement exceeding 3 days over a six-month period.

"In some cases, prisoners were separately confined for months," the statement said.

The United Nations Standard Minimum Rules for the Treatment of Prisoners (the Nelson Mandela Rules) prohibits prolonged solitary confinement.

"Separate confinement practices pose serious risks to the health and wellbeing of the prisoner, and the risk of harm increases the longer the period of separation," the statement said.

The report said it found "strong evidence" of risks related to the "highly damaging impacts of extended separate confinement."

"This evidence was even stronger with respect to risks for Aboriginal prisoners, who make up by far the majority of the prison population in the NT."

One expert told National Indigenous Times all available evidence showed isolation techniques, especially for children, only exacerbated mental health and trauma for people, as well as increase their chance of self-harm and suicide.

Data from the December quarter showed NT imprisoned people at a nation high rate of 1142.8 per 100,000 people; for Indigenous people this sits at 3541.5 per 100,00 people.

Shockingly, almost one per cent of the entire NT population is currently in custody; 87 per cent of which are Aboriginal and/or Torres Strait Islander.

The report found SCP practices "did not appear to be in accordance with legislation, policy and international human rights standards" and the DCC "lacked transparent reporting and recording processes" for proper external oversight.

Furthermore, it said prisoners who were placed in extended periods of separation for any reason had "limited therapeutic or stimulating interventions that would assist their rehabilitation and return to normal prison routine."

In the NT, the two most common forms of SCP are described as 'administrative separation' and 'Intensive Management Plan' (IMP). Both can involve extended, and sometimes repeated, periods of separate confinement; the latter often stretching over long and indefinite periods.

"While administrative separation is confined to a maximum of 14 days, IMPs can last for an indefinite period, measured in months," the report stated.

"Our review revealed multiple IMPs that lasted as long as 3 months. There were also instances where IMPs appeared to last in excess of 5 months."

The ombudsman's report comes after a separate report, released last year by the United Nations anti-torture watchdog, the Subcommittee on Prevention of Torture and Other Cruel, Inhuman or Degrading Treatment or Punishment (STP).

It found inmates at DCC had been kept in solitary confinement for "months at a time in a small cell, without any contact with family or updates on current affairs, and with no television or books."

The use of isolation techniques - which are routinely changed in time by governments - has been heavily criticised for its use on children, with some being held in adult facilities due to factors such as overcrowding.

In March, National Indigenous Times reported on two disabled Aboriginal children dying in the immediate aftermath of being kept in isolation for significant amounts of time in youth detention.

The Australian and New Zealand Children's Commissioners, Guardians and Advocates (ANZCCGA) condemned the use of practice in February and said the current public reporting and accountability for isolation use in youth detention was "inadequate."

"Current definitions, record-keeping and reporting arrangements hide the extent to which isolation is used on children and young people in youth detention, and the ability to monitor progress towards ensuring isolation is only used in strict compliance with international human rights standards," ANZCCGA said.

The Department of Corrections said no person can be kept in solitary confinement in NT, defined as "a punitive form of imprisonment where a prisoner lives in a single cell with little to no contact with other people, or outdoor access."

Despite the ombudsman's report, a Corrections spokesperson maintained separate confinement of a prisoner cannot exceed 14 days, with the "vast majority of separate confinement periods" ceasing within 48 hours.

"Separate confinement is generally used in response to a situational or unexpected incident," the spokesperson said.

DCC has seen 492 prisoners placed in separate confinement since 9 May 2023, and Corrections said all required to be reviewed by a Chief Correctional Officer within 2 hours, with most "removed from the separate confinement the same day."