Peter Dupas was born on July 6, 1953. He was the youngest of three children, born into what was described as "a fairly normal family." He was born in Sydney, New South Wales, but his family moved to Melbourne, Victoria, when he was still a toddler. His siblings were much older than him, and his elderly parents treated him almost like an only child.

Dupas left high school after completing Grade 10 and later got his Higher School Certificate. On October 3, 1968, when he was 15, Dupas went to his neighbor's house, asking to borrow a knife to peel vegetables. Once inside, he attacked the woman, stabbing her in the face, neck, and hand as she tried to defend herself. He was caught and told the police he didn't know why he attacked her and that he couldn't help himself. He was put on 18 months' probation and sent to Larundel Psychiatric Hospital for evaluation. After two weeks, he was released and treated as an outpatient.

In October 1969, a mortuary at the Austin Hospital was broken into, and the bodies of two elderly women were mutilated with a pathologist's knife. One body had a strange wound on the thigh. Police suspected Dupas was involved in this break-in because the wounds were similar to those found on a later murder victim, Nicole Patterson.

Senior Detective Ian Armstrong interviewed Dupas on November 30, 1973, at the Nunawading Police Station. Armstrong described Dupas as "weak and compliant" when confronted by authority. He said that Dupas stood out as someone who was purely evil and showed no remorse for his actions. Armstrong believed that Dupas carefully planned his attacks and would continue to offend, potentially causing the death of one of his victims if he wasn't stopped.

He started his criminal activities at a young age. At just 15 years old, he stabbed a female neighbor and received 18 months' probation for this attack.

On July 25, 1974, Dupas was sentenced to nine years in prison, with a minimum of five years, for a brutal attack on a woman in her home. He broke into her house, threatened her with a knife, tied her up, and raped her. He even threatened to harm her baby if she resisted. The judge described the crime as one of the worst rapes imaginable. A prison psychiatrist, Dr. Allen Bartholomew, noted that Dupas was in constant denial about his crimes and had a serious psychosexual problem, making him potentially very dangerous.

In 1979, just two months after being released from prison, Dupas attacked several women over a ten-day period. On February 28, 1980, he was sentenced to five years in prison for three charges: assault with intent to rape, malicious wounding, assault with intent to rob, and indecent assault. A report from that year described Dupas as extremely disturbed, immature, and dangerous.

Dupas was released from prison again in February 1985. About a month later, he raped a 21-year-old woman on a beach in Blairgowrie, Victoria. He attacked her with a knife, held her down, and raped her. He later apologized to the police, saying he thought he was okay and just wanted to live a normal life. On June 28, 1985, Dupas was sentenced to 12 years in prison for this rape, but he was released after serving seven years in 1992.

Less than two years after his release, Dupas was arrested for false imprisonment after an incident at Lake Eppalock in January 1994. Wearing a hood and armed with a knife, tape, and handcuffs, Dupas followed a woman who was picnicking and held her at knifepoint in a toilet block. Her friends chased him away, and he was caught after crashing his car while trying to escape. On August 18, 1994, Dupas pleaded guilty to false imprisonment and was sentenced to three years and nine months in prison, with a minimum of two years and nine months. He was released again in September 1996 and moved to Pascoe Vale, a suburb of Melbourne.

After his release, Dupas continued his violent attacks, leading to his eventual arrest and conviction for multiple murders. He remains in prison, serving multiple life sentences without the possibility of parole.