The number of Aboriginal and Torres Strait Islanders behind bars has surged by almost 30, and they now make up nearly a quarter of the ACT's jail population in the past year – an increase Indigenous groups have described as "distressing".
Ninety of 413 detainees, or 22 per cent, at the Alexander Maconochie Centre identified as Aboriginal or Torres Strait Islander on January 1 this year, figures from the Justice and Community Safety directorate showed.
That was 29 more Indigenous detainees than the 61 recorded among the jail's 322 prisoners, or 19 per cent of the population, at the start of January 2015.
The figures had worsened by the start of this month, when 102 of the 414 people in detention – or one in four – were Indigenous.
Winnunga Nimmityjah Aboriginal Health Service chief executive Julie Tongs said it was "quite distressing" to think Indigenous members of the community continued to make up such a high proportion of the territory's detainees.
Aboriginal and Torres Strait Islander residents continue to make up roughly less than 2 per cent of the overall ACT population.
Last year the territory government committed to addressing the over-representation of Indigenous in the justice system by vowing to slash the rate of Aboriginal and Torres Strait Islander people in the jail by 40 per cent over five years.
The health service said Indigenous detainees had increased by more than the five-year reduction target set by the government, and it would now need to achieve a drop of about 80 per cent in four years to meet its target.
ACT Corrections Minister Shane Rattenbury said an increase in the number of Indigenous detainees in the territory's jail was in line with an unprecedented rise in prison populations across Australia.
Soaring detainee numbers have put enormous pressure on the country's overcrowded prisons, including the AMC, which has been urgently upgraded to cope.
"It is concerning that the ACT continues to have a higher-than-average representation of Indigenous detainees and that we appear to be having difficulty meeting the targets that have been set," Mr Rattenbury said.
"These targets will require constant vigilance and scrutiny to ensure that we are doing all we can to reduce the trends."
Ms Tongs said more needed to be done to address the causes of high Indigenous incarceration rates, and the government and community needed to find "a new way of doing business" that encompassed health, education, housing and employment.
She said many male Indigenous detainees were in their late teens and early 20s, and often struggled to find enough support once they left jail, which could lead to a long-term cycle of repeat offending.
"It's about prevention, and the only way we can prevent this is to put better supports in place for vulnerable families, especially for people with mental illness and substance abuse problems.
"It's not just a justice problem, it's a whole-of-government problem around employment and housing as well."
Mr Rattenbury said the government was committed to reversing the trend and had taken numerous steps to address the numbers of Indigenous detainees and improve outcomes for them while they were in jail and after their release.
He said that was done through the government's justice agreement with the ACT Aboriginal and Torres Strait Islander Elected Body, and support programs such as the Throughcare initiative for released prisoners.
Indigenous people made up an average of 17.6 per cent of the ACT's jail population in 2012-13, and an average of 17.7 per cent of detainees the next year.
That had jumped to an average of 19.8 per cent in 2014-15, although that figure was still below the national Indigenous incarceration rate of 27.6 per cent.