Alistair Farquhar MacRae, 77, was granted parole in June despite his plea in 1997 that a court show him no mercy and lock him up for the rest of his life.
South Australian Parole Board chair Frances Nelson said the board granted MacRae's parole because "basically, he no longer presents a risk to the community".
"He was a paid hitman, of course, dreadful crimes," she told AAP.
"He put it to us that he had lost his moral compass, and he's ashamed of himself."
MacRae previously declined to apply for parole because "he didn't think he'd been punished sufficiently", Ms Nelson said.
"But more recently, his son has been in contact and persuaded him to apply for release."
A review of the parole approval has been sought under the South Australian Correctional Services Act, which allows the attorney-general, police commissioner or commissioner for victims' rights to request a review.
On Tuesday, SA Parole Administrative Review Commissioner David Lovell will hold a preliminary hearing in his review of the decision.
MacRae shot and killed Dominic Marafiote, 42, at Merbein, in Victoria's northwest in July 1985, burying his body under a chicken coop at MacRae's home.
He then drove to Adelaide, where he executed Mr Marafiote's parents, Rosa and Carmelo, at their suburban Woodville North home.
MacRae's killing spree began about six months earlier at his home in Merbein, when he executed Albert O'Hara, who had come to the property with $10,000 in cash to complete a drug deal.
Ms Nelson said MacRae's prison behaviour had been good, he accepted responsibility for his crimes, had shown considerable contrition and his age had also been taken into consideration.
The parole board had also checked with the SA Police counterterrorism and major crime sections to see if MacRae had a continued association with organised crime, or if he presented a risk to victims of his crimes.
"They reported that they had no evidence of any risk," Ms Nelson said.
MacRae, who has been at the Yatala Prison Pre-Release Centre for several months, was arrested in 1987 and extradited to SA after being found guilty in Victoria of the murders of Mr Marafiote and Mr O'Hara.
When he was sentenced for the murders of Rosa and Carmelo Marafiote in the SA Supreme Court in 1997, MacRae had his non-parole period extended by 10 years, to 36 years.
Before sentencing, MacRae told Justice Horton Williams that to allow him to ever be released would place a bigger value on his life than he had placed on the lives of his victims.
"To allow me leniency is a luxury I did not extend to my victims," he said.
"I would ask the court to give me no possible chance of release before my death in custody."
However, Justice Williams said he was bound to sentence MacRae in accordance with the law and provide him with some chance of release.
MacRae had been involved with organised crime and the drug trade for a long time and had been corrupted by greed and the desire for money, he said.
17°C