A decision to release James Vlassakis by South Australia's Parole Board was upheld by the Parole Administrative Review Commissioner in a judgment made public on Tuesday.
Vlassakis pleaded guilty to four of the 11 murders, which took place between 1992 and 1999. He was aged 18 when he became involved in the killings.
In 2002, he was sentenced to life imprisonment with a minimum term of 26 years.
The order was backdated to his arrest in 1999 and expired in August 2025.
Vlassakis was found to have been heavily influenced to commit the crimes by John Bunting, who along with Robert Wagner was found guilty of the bulk of the murders.
The bodies of eight of their victims were found dismembered in barrels inside a disused bank vault in Snowtown, north of Adelaide, in 1999.
Bunting and Wagner were sentenced to multiple terms of life imprisonment without the possibility of parole.
"Mr Vlassakis, while a young man, came under the influence of Mr Bunting, a person who he looked up to as a father figure," Parole Administrative Review Commissioner David Cameron KC noted in the decision.
"It is clear that this same power dynamic cannot be replicated, given Mr Bunting was sentenced to life imprisonment without parole and that Mr Vlassakis is now of a more mature age.
"It must also be acknowledged that Mr Vlassakis demonstrated excellent behaviour while in custody and there is no suggestion that he displayed any violent behaviour during this time."
SA Attorney-General Kyam Maher said he would review what further steps could be taken to keep Vlassakis behind bars, but noted the decision had already been thouroughly considered by multiple review processes.
"It is traumatic for people involved, the families of victims, of communities in relation to the bodies in the barrel cases," he told reporters on Tuesday.
"It is a very difficult thing for those who are still around who remember what happened so well.
"The gravity and the effect this has had on SA and the need for community safety, were always the primary concerns of the state government."
Mr Maher said those found most responsible for the murders would never be released from prison.
The exact conditions of Vlassakis' release are yet to be decided but could include electronic monitoring and abstaining from drugs and alcohol.
Previously proposed parole conditions would require him to spend a period at the Adelaide Pre-Release Centre, designed to assist offenders with community reintegration.
The Parole Board's decision in August 2025 to release Vlassakis upon the expiration of his non-parole sentence was challenged by Mr Maher and subsequently overturned by then-Parole Administrative Review Commissioner Michael David KC.
That decision was set aside by South Australia's Court of Appeal in June, and a fresh review was ordered.
Mr Cameron, who was appointed to the role in January, on Monday dismissed the attorney-general's application and affirmed the Parole Board's decision to grant Vlassakis' release.
In initially deciding to release Vlassakis, the Parole Board noted he had completed all recommended core programs and only had two behavioural incidents recorded, both of which appeared to be minor.
"Reports indicate that his behaviour was consistently excellent throughout his imprisonment, being described as a quiet, compliant, polite and respectful prisoner," the board stated.
In 2014, Vlassakis was transferred to cottage accommodation within prison which required daily living skills to help prepare him for the outside world.
He tested positive in a urine drug test to methamphetamine in 2017.
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