A study published in Australasian Psychiatry has found that almost two-thirds of NT prisoners have a mental illness.
Substance use disorder was the most common mental illness, impacting almost 70 per cent of female prisoners.
Advocates and experts are calling on the NT government to fund services to support prisoners and reduce reoffending.
A study has highlighted how common mental illness is in the Northern Territory's prison population, despite there being no mental health facility or specialised support.
Every week inside Darwin's Correctional Centre, Danica Winterton and Rob Assan see inmates who need treatment.
They are the backbone of the North Australian Aboriginal Justice Agency (NAAJA)'s mental health team, helping prisoners access medication and lending an ear.
In the Northern Territory, which has the highest incarceration rate in the country, it is life-changing work.
The majority of prisoners in the territory are Indigenous, and many do not speak English as their first or even second or third language, adding to the need for support.
The pair say schizophrenia is the most predominant illness. Substance use disorder is also prevalent.
"There is a real lack of understanding, a lack of empathy or patience is given to that," Ms Winterton said.
But NT prisons have no secure forensic mental health facility or specialised services to help with substance use disorder.
The report by the Royal Australian and New Zealand College of Psychiatry (RANZCP) looked at 310 medical records of inmates across Alice Springs Correctional Centre and Darwin Correctional Centre.
Indigenous people comprised 90.6 per cent of the cohort, despite making up 26 per cent of the general population.
The study analysed data collected in 2024 during three months from men and women who were sentenced or on remand. It included the total population of female prisoners at the time.
Author Sarah Dorrington, co-chair of the Northern Territory branch of RANZCP, said the study was the first of its kind, and supported what was common knowledge.
The researchers discovered that almost two in three prisoners had a lifetime diagnosis of a mental illness, compared with 43 per cent of the population.
"It's a really important reminder that healthcare interventions in custody really need to be prioritised," Ms Dorrington said.
To determine the lifetime prevalence among prisoners, the research team analysed whether inmates had been diagnosed with a mental illness in the past, if they had been admitted into a psychiatric ward or prescribed medication.
"It was surprising based on my experience and the international literature … we think it is really important," Dr Dorrington said.
The study also found close to 80 per cent of the women in NT prisons had been diagnosed with a mental health condition.
"It's very much a symptom of psychosocial disadvantage, poverty, trauma, being exposed to substance use," Dr Dorrington said.
It is reflected in what NAAJA mental health workers also see.
"Every one of the females that we work with has had their children removed, and then they're grappling with that trauma," Ms Winterton said.
Another finding Dr Dorrington found from the research was the "very high" prevalence of substance abuse disorders.
Nearly 70 per cent of female prisoners had been diagnosed with substance use disorder, and just over half of all male prisoners.
Ms Winterton said "dual diagnosis centres" designed to treat people with multiple mental illnesses could make a big difference, for people who had just left prison.
"Residential rehabs don't accept anyone with mental health conditions, because they're not trained, and the mental health services … don't accept anyone with a substance use disorder," she explained.
"All of [our female clients] are homeless, diagnosed with schizophrenia and have substance use disorders, and they're just bouncing between services who are blaming one another.
"And then you throw in the child protection system, you throw in all the vulnerabilities of living on the street and being a woman and being susceptible to sexual abuse.
The study also found 29 per cent of female inmates had a diagnosis of PTSD, which Dr Dorrington said was "an underestimation."
Rob Assan, who works alongside Ms Winterton, said there was still a "stigma with mental health" in First Nations communities.
That's where he sees family and culturally appropriate services playing a big role.
"It takes a long time for our mob to trust," Mr Assan said.
"The biggest thing would be [an Aboriginal] person to go in and sit with them and explain what mental health is, why they're taking the medications, all that stuff that should be done in any clinic," he said.
Dr Dorrington called for a holistic approach to reduce cycles of reoffending.
"Putting people in prison doesn't make them better; in fact, it makes their mental health worse," she said.
"[If you] treat the causes of [mental illnesses], including the homelessness and psychosocial disadvantage, then they are less likely to offend in the future, and ultimately our community becomes safer."
The ABC has contacted the NT government for comment.