Police turned up during his 16^th birthday party to say his 18-year-old sister, Liz, had not come home the night before in Canberra. They wanted to know if Alistair's father, Norm, had heard from her.

Alistair says he knew, even then, something was very wrong.

"From that point I had entered into a kind of catatonic state from which I don't think I have ever quite really emerged," he says.

Liz's sister, Janie, was 21 at the time and says there are no words to describe the infinite sense of loss in the years since.

"Sometimes I just sort of feel like I should be out there, like, hunting down people and finding out what happened to her and getting to the bottom of it. But it's like, there's nothing to go on. There's nothing to do. There's nothing I can actually hang on and nothing I can get any traction with," she says.

"So I feel like I've let her down."

Saturday marked four decades since Liz went missing but Alistair and Janie have not given up hope. Not a day passes where they don't grieve for their kind-hearted sister who was level-headed beyond her years.

Growing up, Liz and Janie were always close - their shared bedroom was a shared world. And for Alistair, Liz was his mentor.

"We were kind of like each other's survival in many ways," Janie says.

Alistair says they just want a conclusion. "We want a resolution. And we can't rest until we have that," he says.

In 1980, Elizabeth Herfort had been back in Canberra seeing family over the June long weekend. She had come down from Queensland where she was picking fruit with her boyfriend and was staying with her mother, Anne, in Pearce.

Liz, who had left school at the end of Year 10, had spent a few years travelling around Australia with her boyfriend. The pair regularly returned to Canberra.

About 2.30pm on Friday, June 13, 1980, having withdrawn her last $1.50 from the bank, Liz took the bus to the ANU Bar, where she spent the afternoon with old school friends, slowly drinking and playing pool.

A young man from the group asked Liz how she was getting home about 8.45pm. She said she was not sure because she had no money. He was getting a bus and offered to buy Liz a ticket, so he assumed they would walk back to the city interchange together.

He waited outside but Liz never came back out. Janie says she probably ducked out from the toilets to another entrance which took her straight outside.

It was the last confirmed sighting of Elizabeth Herfort.

Liz's mum, Anne, raised the alarm the next day, which prompted a two-week search for traces of Liz. Police and State Emergency Service volunteers searched the shores of Lake Burley Griffin but found nothing.

Several witnesses said they saw a young woman trying to hitchhike. A man came forward saying he had seen a man arguing with a young woman by the side of a parked car on Commonwealth Avenue just north of the British High Commission about 9.30pm.

A separate man said he had given a woman matching Liz's description a lift along Commonwealth Avenue from Vernon Circle to the intersection with Coronation Drive.

Police focused their attention on the man seen standing by the car, interviewing him multiple times. He always insisted he was at home and the story was fabricated. The investigation stalled.

Janie and Alistair would meet with police every year to ask when an inquest would be held, but they would have to wait until the law was changed to allow for an inquest where no body had been found.

After the law changed, an inquest was ordered by ACT Coroner Ron Cahill in 1993 which would be paused twice after new evidence was brought forward. The evidence - which included a man apparently boasting of his involvement in abducting Liz, a story he claimed he made up - was ultimately discounted.

In November 1996, Cahill returned an open finding, suspecting Liz had met with foul play.

Janie and Alistair say they have been frustrated by the investigation, which they say was quick to focus on a suspect in the early years and ignore other possibilities. A 2015 police review of the case got nowhere, but a police spokeswoman encouraged anyone with information to come forward and said unsolved homicide cases were never closed.

Janie says she has experienced strange encounters with people she didn't know offering bizarre explanations for Liz's disappearance.

Not knowing what happened for certain meant being open to the exploitation of people who feel they can say anything, she says.

While waiting to speak with police several years after Liz went missing, Janie says a man who looked like a detective turned around to speak to her in the police station foyer.

"He said to me, 'Oh, you're Elizabeth Herfort's sister, are you?' And he said, 'I'll tell you a story of, you know, this story that I know about of some other place where police actually were involved in the abduction of a girl and killed her and there was a cover up'," Janie says.

"And I've never known who he was or what that meant, but that's one of the little encounters we've had."

Alistair says living with the disappearance of his sister has left a lasting scare on him and his family, an indelible mark of sadness.

"I feel a part of me is left back there and hasn't come along. I've moved along. Part of me is left back there and only half of me is operating. I haven't fully matured, I haven't fully emotionally matured. It's quite debilitating," he says. "I used to hitchhike after Liz disappeared in the hope that I would meet her assailant, you know. What was I thinking?"

Both Janie and Alistair say keeping people - and the police - interested in the case 40 years on is a gruelling obligation. Resting would be conceding failure.

"It feels like, to me, giving up is like saying that I'm willing to accept that I live in a world where girls can just disappear and we just go, 'Oh well, they're gone. Dunno'," Janie says.

"And I'm not willing to live in that world at all. That's not a world that I can make sense of."