Valerie Tomkins wants the original finding of the ACT Coroner that the death was an accident to be reconsidered.

That requires a formal application to the chief coroner, which, if agreed to, could lead to a fresh inquest into Troy's death.

Ms Tomkins said she and daughter Vanessa Forsyth had kept quiet for too long about what they claim was a "grossly incompetent" investigation after Troy's death in 1987.

"This is for my children, especially my eldest daughter," Ms Tomkins said.

"It ruined all our lives, none of us are the same, but she was particularly traumatised, and it changed her whole outlook on life.

"There's no revenge, there's nothing that would make it easier for us, but if it could give her some form of comfort, she desperately needs it."

The family, who are now seeking legal representation, were frustrated by delays with the early stages of the investigation.

Ms Tomkins said the case, handled mainly by the Australian Federal Police's traffic police and Criminal Investigations Branch, was mismanaged and she feared there had been either a cover-up or that the suspected driver was let off after giving information on criminals.

"They were conspicuous by their absence, there was no contact or anything," she said of the investigators.

"I went to traffic investigations and saw a young constable who indicated a stack of files on his desk, and he said to me he was lucky if he could even spend an hour a day on my son's case. I was absolutely appalled and extremely distressed, and that prompted me to put in an internal complaint with the commander of internal investigations."

Ms Tomkins' complaint was investigated by the force's internal investigations division, and by the Commonwealth Ombudsman's office.

Both agencies found the complaint to be unsubstantiated and defended police efforts, saying a vast amount of resources had been devoted to the case.

The internal investigations division wrote to Ms Tomkins in November 1987, saying they were satisfied with police efforts.

"I am of the opinion that this investigation has been properly conducted, in accordance with well established procedures designed to ensure that all evidence is gathered for submission to the Coroner; with the objective being for the person/s responsible for the tragic death of your son being brought to justice," the internal investigators wrote.

The family is considering submitting another complaint to the ACT Ombudsman about the failure to retest suspect vehicles after police obtained new forensic information on the colour of the offending van.

Ms Tomkins has renewed pleas for members of the public to come forward with any information on Troy's death.

"Canberra being Canberra, and at that time, there is definitely some information that's being held back," she said.

"Canberra was too small a community, even off-duty policemen have been known to say they thought it was a deliberate act. I know that there are always rumours and innuendo surrounding any sort of criminal activity, but this is just too much."