A government letter shows the NSW police task force established in 1993 to investigate Milat's crimes had compiled a list of 16 victims of unsolved homicides from as early as 1971 that could be linked to the killer.
The victims named were found in Queensland, WA, the ACT, Victoria and NSW.
Milat died in 2019 while serving seven consecutive life sentences for murdering seven backpackers between 1989 and 1992. Their bodies were found in NSW's Belanglo State Forest.
The victims named in the letter include Robyn Hoinville-Bartram, who went missing in Queensland with her friend Anita Cunningham while hitchhiking in 1972.
Ms Hoinville-Bartram's body was found under a bridge about 80km west of Charters Towers, but Ms Cunningham's remains have never been found.
Her brother, David Cunningham, said he had long believed Milat might have been involved in his sister's presumed murder after he was identified as one of a number of suspects in a book on the case by former Queensland detective Michael Gurn.
But he said police had not interviewed any of those suspects and had discounted Milat's potential involvement.
"After 50 years, the chance of finding and getting enough evidence to convict somebody for a murder that long ago is like a snowball's chance in hell," he told AAP.
Another victim named was 20-year-old Keren Rowland, who was five months pregnant when she was found dead after going missing from Canberra in 1971.
A NSW parliamentary inquiry into unsolved murders and missing persons cases, which resumes on Thursday, previously heard Ms Rowland's family had also long suspected Milat might have been responsible for her murder.
But the inquiry's chair, Jeremy Buckingham, said the letter from the NSW Crown Solicitor's Office revealed that while police shared the suspicion, they did not take adequate action.
"In terms of Keren Rowland or the murders in Queensland, Victoria, WA, (NSW Police) say this is outside of our jurisdiction," he told AAP.
"You've got the possibility a known murderer is living in your state, murdering people in other states, and you do nothing about it.
"It beggars belief."
NSW police declined to comment as it was the subject of the ongoing inquiry.
Mr Buckingham said the committee was still waiting for police to provide further information on the Milat investigation, including his employment records, full criminal charge history and the disappearances the task force linked to the killer.
He said he believed the force was withholding information because it was a "damning indictment on police at the time".
"We're talking about the possibility that scores of people have been murdered, and the murderer or murderers have not been brought to justice," Mr Buckingham said.
"And that Milat's career as a homicidal maniac could have been snuffed out … in the 1970s if the police had been doing their job."
Mr Cunningham called for unsolved missing person and murder cases potentially related to Milat to be treated in the same way by authorities.
"Ever since the girls disappeared, as soon as Robyn's body was found, Robyn became a homicide case, and Anita was a missing persons case," he said.
More than 50 years after his sister's presumed death, he said his priority was finding her remains and bringing her home to Victoria.
"Whether Ivan Milat was (a suspect), and I strongly suspect he was, it doesn't bring my sister back," he said.
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