The loved ones of six women who were murdered in a string of killings in the 1980s have spoken of their anguish, as Victoria Police offers an unprecedented $6 million worth of rewards to find the serial killer responsible.

Police hope the six $1 million rewards will help them find the person responsible for the murders of the women killed in what was known at the time as the Tynong North murders.

The women were killed between May 1980 and November 1981, with the victims aged between 14 and 73 years old.

Police said all the women were on foot when they were taken, with the majority intending to take public transport.

Their bodies were found hidden in scrubland at Frankston, in bayside Melbourne, and at Tynong North, south-east of Melbourne, and the offender tried to hide their identities by taking their personal items.

The six victims of a 1980s serial killer were taken while walking around Frankston and Tynong North. ( Supplied: Victoria Police )

Victoria Police Homicide Squad Detective Inspector Mick Hughes said police believed one of the victims, Bertha Miller, had been approached by a man at a tram who had noticed she was carrying a bible, two weeks before her death.

"She spoke to a friend after that and was pretty encouraged, she saw that as an opportunity to talk Christianity to that person," he said.

"We've never identified who that is, and we're working on the principle there's a good chance he is our offender.

"If we look at all of the victims, they've either been using public transport or close to, so he's clearly seen a way in to talk to Bertha, because everything I'm told about her, she wouldn't engage with people unless there was a reason to do so."

'Terrible damage to families'

Keith Rooke, the son of Allison Rooke, who was the first of the women to fall victim to the killer, said bringing the killer to justice would help his family to talk about his mother.

"We've had a traumatic time and it's a sad way to lose our mum — a mother of six — and she was a great mum and now everyone's had to move on, it's a lot of time, but still we would love to see some finalisation," he said.

"It would also allow us within our own family, children, grandchildren, great grandchildren, to talk about it and make sure that our mum's not remembered just for what happened, but remembered for the great person that she was."

Cold case murder victims: Allison Rooke, 59, was last seen leaving her Frankston North home at 11:00am on May 30, 1980, to catch a bus to the Frankston shops. Her body was discovered by in Frankston scrubland by a man walking his dog on July 5, the same year.

was last seen leaving her Frankston North home at 11:00am on May 30, 1980, to catch a bus to the Frankston shops. Her body was discovered by in Frankston scrubland by a man walking his dog on July 5, the same year. Bertha Miller, 73, was last seen leaving her Glen Iris home on the morning of August 10, 1980, to catch a tram to a church in Prahran.

was last seen leaving her Glen Iris home on the morning of August 10, 1980, to catch a tram to a church in Prahran. Catherine Headland, 14, was last seen leaving her boyfriend's house in Berwick on August 28, 1980 to catch a bus to Fountain Gate shopping centre in Narre Warren.

was last seen leaving her boyfriend's house in Berwick on August 28, 1980 to catch a bus to Fountain Gate shopping centre in Narre Warren. Ann-Marie Sargent, 18, was last seen leaving her mother's Cranbourne home at 9:30am on October 6, 1980 to catch a bus to Dandenong. The bodies of all three women were found on December 6, 1980, after a group of men dumping animal remains in scrub in Tynong North came across human remains and contacted police.

was last seen leaving her mother's Cranbourne home at 9:30am on October 6, 1980 to catch a bus to Dandenong. The bodies of all three women were found on December 6, 1980, after a group of men dumping animal remains in scrub in Tynong North came across human remains and contacted police. Narumol Stephenson, 34, was last seen outside a friend's house in Brunswick in the early hours of November 29, 1980. On February 3, 1983, a man discovered a human bone sticking out of a bush when he stopped to change a flat tyre in Tynong North, leading to the discovery of Ms Stephenson's body.

was last seen outside a friend's house in Brunswick in the early hours of November 29, 1980. On February 3, 1983, a man discovered a human bone sticking out of a bush when he stopped to change a flat tyre in Tynong North, leading to the discovery of Ms Stephenson's body. Joy Carmel Summers, 55, was last seen at a bus stop in Frankston at about 1:00pm on October 9, 1981. Her body was found in scrubland in Frankston North on November 22 the same year.

Ms Rooke was missing for more than two months before she was found.

"When eventually her body was found, because of the way the investigation has got to go, it tears families apart, it puts a lot of pressure on families and you have to make a very conscious effort to get over that, and move on," Mr Rooke said.

"It does terrible damage to families."

Police said they had spoken to more than 2,000 people over the murders and a number of persons of interest were identified.

A new taskforce was established in 1998, but no-one has ever charged over the deaths.

The brother of Ann-Marie Sargent, Peter Sargent, praised the police for their continued efforts to bring the killer to justice.

"I think the police have done a great job, because they've not parked away and just forgotten about it, over the years it's brought up every now and again and they're still working on it," he said.

He said his sister was bubbly and loved kids, and despite all the years that had passed, the family had been "emotionally wrecked" by her death.

"Even now, we are hopeful that it will get solved … because the pain for us is really still there on a day-to-day basis in the background, but especially when we have events like Christmas and birthdays, we miss her being there.

"It's not going to go away for us, it's not going to bring her back, but at least we'd know that it's been solved."

'We thought we were safe'

Cheryl Goldsworthy, the best friend of 14-year-old victim Catherine Headland, said the loss of her friend, who she "idolised", changed her life.

"It changed us all … we thought we were safe. There was a group of us and we always stuck together, basically climbing over the back fences at night because you don't go out at night," she said.

"It shocked me and shocked us all. It still does. It dictated how I raised my children, and I know it also had the same effect on her other friends and their daughters.

"I wouldn't let my girls go out, I wouldn't let them go on a bus because it was broad daylight and it just should never happened, she just had to cross a street."

She said she "truly believed" someone out there knew something about what happened to the women.

"The more that it's brought up, and the more that it's publicised, and the more people that know about it, sooner of later somebody's going to say something and put it to rest for us," she said.