The 30-year-old, who The Canberra Times has not named, was on remand over allegations he assaulted his girlfriend in Civic and obstructed police in full view of the public one night in early June.

He was released the next day, but was locked up in the Alexander Maconochie Centre on June 14, after he breached his bail conditions by going to his girlfriend's house in Weston.

The Lyneham man was marked as a prisoner at risk when he was sent to the jail, and his mental health issues had been noted in earlier, unrelated court proceedings. Once inside the jail, he should have been placed under strict observation, either on intervals of five, 15, or 30 minutes, according to corrections management policy, although it is understood such frequent checks are often impractical.

The last check on the man occurred less than an hour before his body was found about 7pm, police said. He was not being kept in the prison's crisis support unit - where those at the highest risk of suicide and self-harm are kept.

He was being held in a cell in the prison's health centre, which The Canberra Times understands is not typically staffed at night, and is not intended to hold those who are feared likely to self-harm or commit suicide.

It is unclear whether the inmate was still marked as a prisoner at risk when he killed himself on Tuesday, and Corrections Minister Shane Rattenbury refused to answer further questions on the issue on Wednesday evening.

A prisoner's suicide risk is routinely reassessed, and prison health staff may have removed or downgraded the man's observation regime before his death.

Criminal investigations detectives are probing the death, while Corrective Services will also conduct an internal review.

Prison officers attempted to resuscitate the man when they discovered his body shortly before 7pm, but were unsuccessful.

Mr Rattenbury told a press conference on Wednesday morning that a report would be prepared for the ACT Coroner.

''On behalf of the ACT government I would like to express my deepest sympathy to the family and friends of the individual,'' Mr Rattenbury said.

He said corrections and health staff were being offered support, as were detainees.

Criminal Investigations Detective Superintendent Brett McCann described the death as ''tragic''.

Forensic examinations have been conducted, and a post-mortem was likely to be conducted on Friday.

Police expect to have their brief ready for presentation to the coroner in a couple of months.

This is the first suicide in the AMC's short, four-year history, with only one other death in custody, which was due to natural causes.

Between July 1979 and June 2011, a total of five deaths of individuals in ACT Corrective Services' custody have occurred.

In 2008, troubled psychologist Tania Lioulios tried to asphyxiate herself with a seatbelt in the back of a Corrective Services van transporting her from the ACT Magistrates Court.

The officers transporting her to the Belconnen Remand Centre were not told she was marked as a prisoner at risk. The woman, who later died from her injuries, was left locked and unsupervised in the van.

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