In a message sent less than 24-hours before his death, the 29-year-old man told his girlfriend he was being locked back in his cell, and hoped she would have a good night.

"I'll email ya when thay [sic] let me out 2morrow k beautiful love ya," the message read.

The girlfriend said he knew he deserved to be there but was looking forward to coming home.

"He was eligible for parole in 18 months and he couldn't wait to get out," she said.

"As much as he hated being in there and being confined, he looked at the bright side of things. He wanted to start a better life."

ACT Policing are investigating the second death at the Alexander Maconochie Centre within 12 months. Prison staff tried to revive the man after a cell mate raised concerns about his condition at 7.30pm. Police said he died roughly half an hour later.

The ACT government and police said they could not provide further details of the death while investigations continued.

Fairfax Media accepted the family's request not to name the man at this stage. His partner of two years also asked to remain anonymous due to risk of identifying him.

The woman said she was still in shock.

"I still can't believe it has happened, I am still waiting for him to call me."

"He was always there for me no matter what. I am angry and frustrated. Something has gone terribly wrong somewhere."

The man's sister was understandably devastated and asked for the family's privacy to be respected.

ACT justice minister Shane Rattenbury said the death was a tragedy and very unsettling.

"There is not a whole lot more we can say, as the autopsy won't take place until tomorrow and we don't have any evidence yet," he said.

"It can be frustrating for members of the public who feel they don't have all the appropriate information, but things will go through the processes they need to and it will become available when it can."

In a statement sent on Sunday night, the Canberra Liberals were quick to link death to the significant systemic failings that "call into question the government's duty of care for inmates".

Shadow justice minister Giulia Jones called the incident an abject failure in the administration of justice.

Mr Rattenbury slammed those comments as extremely premature as the cause of death was not yet known. "They politicised the issue within 24 hours of the death when the family had only just been notified," he said.

Ms Jones said she would wait for the coroner's findings but that the chances of it being a natural death were "extremely low".

"The coronial investigation will shed some light on what the profile of this young man was," she said.

"I think everyone is shocked that we have had another person die in our prison. People are sentenced to be in the prison, but it is not a death sentence."

Support group Prisoners Aid ACT expressed its condolences to the man's family.

The death followed the death in custody of Indigenous inmate Steven Freeman in May last year, which prompted an independent inquiry that found significant flaws in the prison system.

An inquest into the 25-year-old heard he was admitted to the prison methadone program two days before he died, despite no record he was ever using heroin.

Six prisoners have died in custody since the ACT prison opened in 2009: one suicide, three from natural causes, Steven Freeman and the most recent death.