The Gomeroi teenager was found dead eight kilometres south of Tamworth in the state's northwest on January 16, 1988.
On the 35th anniversary of his death, his family are calling for a new coronial inquest, saying it will give them some closure and help other Indigenous families navigate the legal system.
"After all the years of not knowing what happened to my brother Mark, it is still overwhelming and raw that no justice has come of it," said Mr Haines' sister Lorna, who was 15 when he went missing.
"My mother and father went to their graves not knowing anything about what happened to their son, which still leaves scars with myself and (my brother) Ron."
Lawyers from the National Justice Project, who are assisting the family, said police investigations into Mr Haines' death were inadequate. A 1989 inquest returned an open finding.
Solicitor Karina Hawtrey said a new inquest could examine the cause of Mr Haines' death and expose flaws in the police work.
Ms Hawtrey said police found a stolen car crashed near Mr Haines' body and concluded he laid on the tracks either deliberately or in a dazed state, something his family never believed.
"Mark couldn't really drive and so how he's been able to steal the car, drive it, crash it: there were some serious unanswered questions," Ms Hawtrey told AAP.
"For an Aboriginal family in Tamworth in the 1980s, feeling like the police weren't taking their concerns seriously, they felt the police were being quite racist in the way they were dealing with this case."
NSW Police renewed an appeal into Mr Haines' death in 2018, offering $500,000 for information.
Homicide detectives handed a review of the case records to the coroner last year, while Tamworth police continue to investigate under Strike Force Puno.
"The Oxley Police District are determined to find answers about what happened to Mark and to bring closure for his family," Superintendent Bruce Grassick said on Monday.
"The case remains open and any information that comes to light will be given the highest priority and fully investigated."
Mr Haines' family gathered outside Tamworth police station on Monday morning to make their pleas public.
His uncle, Don Craigie, said it had taken too long to find answers.
"We maintain that Mark has met with foul play," he said.
"That was our position 35 years ago and, subsequently, nothing has changed our minds. What we are seeking is the truth of what happened to Mark."
