Evelyn Greenup, 4, Clinton-Speedy Duroux, 16, and Colleen Walker-Craig, 16, went missing from Bowraville, south of Coffs Harbour, during a five-month period from 1990 to 1991.
Two of the Indigenous youths' remains were found in nearby bushland in 1991, but Colleen's body has never been found.
Finding her remains is a key priority of the police's latest investigative push, with the force repeating calls on Monday for anyone with information about the disappearances to come forward.
Detectives will be at a local hall in Bowraville until Friday as they appeal for community members to help.
Colleen's clothes were found in the Nambucca River after her disappearance and the NSW coroner ruled she was most likely murdered.
Detective Superintendent Danny Doherty says police have not given up on solving the case. (Mark Evans/AAP PHOTOS)
Homicide Squad commander Danny Doherty said the police presence in Bowraville should let the community know detectives had not given up on finding who killed the trio.
"One of the most heartbreaking aspects of this case is that Colleen's remains have never been found and that will be a key focus of our detectives this week," he 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."
A man, who can't 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 that was her definitive cause of death.
A $1 million reward remains for information that leads to an arrest and conviction.
