''I knew then that she hadn't just slept out the night,'' Mr Herfort said.

''I knew she wasn't coming back, I just had that feeling.''

More than three decades after Elizabeth Herfort vanished, her family still doesn't have a clear picture of what happened to the carefree teenager.

They have also given up all hope she will ever be found alive.

It was the end of the dole fortnight when Elizabeth and a group of friends gathered to spend their last few dollars on a couple of jugs of beer at the ANU Bar at the Australian National University campus on June 13, 1980.

The 18-year-old planned to catch the bus home with a friend that night, but apparently decided to hitchhike from Civic, perhaps because she did not have enough money for the fare.

The last known sighting came from a man who told police he saw a woman who matched Elizabeth's description speaking with a man in a car on Commonwealth Avenue, opposite the Hyatt Hotel Canberra.

Mr Herfort, of Queanbeyan, believes one or both of those men were involved in his sister's disappearance, or know her fate.

''She was a bit of a hippie and that's why she hitchhiked. It's just what you did in those days,'' Mr Herfort said.

''She was free-living and spiritual. She was a beautiful person.''

''When she left, she left a huge hole, a huge vacuum.''

A coronial inquiry in the late 1990s concluded Elizabeth was most likely dead, probably as a result of foul play, and that there was not enough evidence to commit anyone to trial.

Elizabeth's relatives will grieve with other families who have had a loved one disappear on International Missing Children's Day on Sunday.

''But hopefully unlike the families of other missing people, who we hope their loved ones come home, we don't expect that any more,'' Mr Herfort said.

''We know she's not coming home and that's our reality.''

Mr Herfort said his family's struggle with his sister's disappearance had been ''very black and very isolating''.

''The loneliness, the sadness, the grief. It's very private and very personal,'' he said.

''We'll always grieve. The saying is that time heals all wounds, but the pain just gets obscured under the noise of life. It's always there.''

Alistair Herfort urged members of the public to come forward if they had information about any missing persons case, although he was convinced it was too late for his sister.

''It was two weeks after the police appeals for Liz for any witnesses to come forward,'' Mr Herfort said.

''It's critical the timeliness with which people come forward to the success of finding a missing person.''

Rebecca Kotz, team leader of the Australian Federal Police's National Missing Persons Coordination Centre, said even the tiniest piece of information could be the final piece of the puzzle which helped crack a case.

She said International Missing Children's Day was an opportunity to shine a light on the 17,500 children reported missing each year.

The vast majority were found alive and well soon after their disappearance. However, Ms Kotz said about 1 per cent vanished without trace.

''I urge anyone, if you have any piece of information, no matter how minute you think it is, to come forward.'' she said.

''And if someone goes missing and there's concern for their welfare, don't muck around. Report it right away.''

Information about missing persons can be reported to Crime Stoppers, 1800 333 000. For more information visit missingpersons.gov.au.