On 19 August 1992, Simpson disembarked from her school bus. Her mother, who usually met her at the bus stop, was busy that day and had arranged for Simpson's older brother to meet her and accompany her home, after his bus reached the bus stop. But his bus arrived later than usual and Simpson was not there. Simpson, not seeing her brother at the bus stop decided to start the walk home, which was only a short distance away, and see him at home.
Police immediately suspected that Simpson had been kidnapped rather than running away. Suspicion fell on a man who was seen working on his car near where Simpson had gotten off the bus. Over a hundred people began searching the area for Simpson, including police, firemen, State Emergency Service members, and volunteers.
On 21 August, police found Simpson's body in a dam at a wildlife sanctuary near her home. Her hands and feet were still bound. Later that day, Garforth was arrested and confessed to the murder. With her house in sight, Garforth had grabbed Simpson, thrown her in the boot of his car and drove off to a remote dam. Once there, he bound her with wire, raped her, weighted her schoolbag and threw her into the dam's reservoir, where she drowned. According to police, Garforth had participated in the search for Simpson on the day her body was found.
Garforth confessed to the crime after police detained him, showing no remorse for his actions during the confession and court sessions. He pleaded guilty to the murder of Simpson and was sentenced in 1993 to life imprisonment. Justice Peter Newman refused to fix a non-parole period and ordered that Garforth's papers be marked "never to be released".
Garforth appealed to the High Court of Australia, but he was refused special leave. It is one of two similar cases which were refused special leave. In discussing the meaning of "life imprisonment" when Garforth appealed his sentence the Judges said, "the community interest in retribution, deterrence, protection of children and the community in such situations may so strongly outweigh any regard for rehabilitation that a life sentence becomes the only option."
In 1995, Garforth lodged several claims for victims' compensation via his lawyers, relating to alleged assaults which occurred in prison. The claims were later withdrawn after public outrage. Garforth's prisoner status was downgraded upon the recommendation of the Serious Offenders Review Council in 2015, giving him access to prison employment and rehabilitation courses. The council's decision was immediately reversed by Minister for Corrections David Elliott.