Evelyn Greenup, 4, Clinton Speedy-Duroux, 16, and Colleen Walker-Craig, 16, went missing from Bowraville, south of Coffs Harbour in northern NSW, during a five-month period beginning in 1990.
Two of the Indigenous youths' remains were found in nearby bushland in 1991, but Colleen's body has never been found.
Her clothes were found weighed-down in the Nambucca River and a coroner found she had most likely been murdered.
Finding her remains is a key priority of the police's latest investigative push.
Colleen's mother Muriel Craig joined the appeal for information that could bring closure on Monday.
"I know that there's people out there who know things ... don't you think it's time that you told me," Ms Craig said.
"I want to know, I don't want to leave this earth, not without knowing where she is.
"I just want people who know things just to come forward, if you don't karma will get you anyway," she said.
Detectives will be appealing for community members to help at the hall in Bowraville until Friday, where a missing-person poster advertises the $1 million reward on offer for useful information.
The presence was an important symbol to the community police had not given up on finding a killer, Homicide Squad commander Danny Doherty said.
"Finding Colleen's remains is absolutely critical in helping us close this painful chapter and we are hopeful there are still people out there who have information that could point us in the right direction," Det Supt Doherty said.
Returning to Bowraville showed investigators remained invested in finding answers more than 34 years after Colleen disappeared, he said.
"It's a small gesture to be here but it's an important one, because people can come and talk to us, we're trying to build trust again."
Information can also be given through Crime Stoppers and investigators would visit potential sources who could not make it to the hall, Det Supt Doherty said.
Homicide Squad commander Danny Doherty says the Bowraville visit is a small but important gesture. (HANDOUT/NSW POLICE)
A man, who cannot be named for legal reasons, was acquitted of Clinton's murder in 1994 and of Evelyn's murder in 2006.
The government in 2018 unsuccessfully argued in the NSW Court of Criminal Appeal there was fresh and compelling evidence - related to the disappearance of Colleen - that justified overturning the two acquittals and launching a fresh, single trial on three murder charges.
The High Court refused to grant special leave to the NSW government to appeal the decision.
An autopsy found Evelyn had a skull injury consistent with forceful penetration by a sharp instrument, but stopped short of declaring a definitive cause of death.
No one has been charged with Colleen's murder.
A parliamentary inquiry into family responses to the murders in Bowraville recommended reviews of procedures and training on Aboriginal cultural awareness, as well as funding for memorials for the children in 2014.
A million-dollar reward is in place for information relating to the disappearance of each child, which extends to locating Colleen's remains.
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