The family of Keren Rowland, who was found dead after disappearing one night in the early 1970s, believe she may have been the first victim of the notorious serial killer.
They argue if police had made the connection early on, he could have been stopped much earlier in his lethal campaign.
Ms Rowland's younger brother Steve appeared on Thursday at a NSW parliamentary inquiry into unsolved murders and missing persons cases between 1965 and 2010.
"We have and will continue to search for answers, but we're finding it incredibly difficult to get the same support and help from the investigating authorities," he told the committee.
Ms Rowland disappeared on February 26, 1971 after spending the evening at the Canberra Show. She was aged 20 and was five months pregnant.
Retired detective Hugh Hughes, a former British police officer married to Ms Rowland's cousin, described being knocked back repeatedly from receiving archival information from the Australian Federal Police as "institutional corruption".
ACT Police investigated the case, followed by the AFP
"It means the current organisation and its people trying to cover up errors they made in the past," Mr Hughes said.
"Unfortunately, as it stands today, I can't rule that out.
"Police need to learn to be open, honest and transparent and own mistakes ... the families deserve better, Australia deserves better."
Milat died in 2019 while serving seven consecutive life sentences for murdering seven backpackers whose bodies were found in NSW's Belanglo State Forest in the 1990s.
Legalise Cannabis MP Jeremy Buckingham, who is chairing the inquiry, said he believed Milat's murders dated back to the early 1970s and involved several victims whose cases remained unanswered for decades.
Ms Rowland's family believe her abduction and murder was sexually motivated and may have been the start of Milat learning his predatory behaviours.
The family say at numerous junctures police could have connected her death with the serial killer.
The state's top homicide cop, Detective Superintendent Joe Doueihi, acknowledged the pain and the loss of the families taking part in the inquiry.
When asked about a potential Milat connection to the cases, the senior officer said there were three unsolved homicides in which the serial killer may have been involved.
He did not provide further details to the inquiry.
Det Supt Doueihi noted a team of 36 officers in the Unsolved Homicide Unit were tasked with investigating 700 cases.
The painstaking work was a "really slow burn" in comparison to ongoing homicide cases, to which more than a dozen officers might be assigned at a time, he said.
Choking up at times during his testimony, Kevin Docherty, whose twin sister Kay disappeared from the Illawarra region 47 years ago, touched on how his family was forced into playing detectives themselves.
"This was not our job, but families do this for their loved ones," he said.
He also believes there could be a connection with Milat based on witnesses who had since come forward and the killer working only 15 minutes away from Warilla, where the twins grew up.
Jeff Dakers, a former detective turned pastor who worked on the case, criticised the force for "bureaucracy, complacency and just lack of interest".
Missteps by investigators in the cold case of toddler Cheryl Grimmer, who disappeared from a beach near Wollongong more than five decades ago, were also raised in the inquiry.
"We're not asking for your sympathy, we are asking for transparency, we are asking for accountability," her brother Ricki Nash said.
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