David and Catherine Birnie

22-year-old Mary Neilson was studying psychology at the University of Western Australia and working part-time in a delicatessen when she met David Birnie at the spare parts yard he worked at. David offered to sell her cheap tyres for her car, and subsequently gave her his phone number.[6] On 6 October 1986, she went to the Birnies' house.[6] She was gagged, chained to the bed and raped by David while Catherine observed. She was taken to Gleneagles National Park near Albany Highway in Bedfordale where she was raped again and strangled with a nylon cord. He then stabbed her thinking that it would speed up the decomposition as he "read that in a book somewhere", and they buried her in a shallow grave. She would have received her degree for psychology from the university one year after her murder.

Two weeks after the murder of Mary Neilson, they abducted 15-year-old Susannah Candy as she hitchhiked along Stirling Highway in Claremont, Australia. She was an outstanding student at Hollywood Senior High School, and lived at home with her parents and siblings in Nedlands, Australia.

Her father is one of the top ophthalmic surgeons in Western Australia. After she went missing, the Birnies forced her to send letters to her family to assure them that she was all right. But the family feared for her life.

The Birnies had been cruising for hours looking for a victim when they spotted Candy. Once she entered the car she was held at knife point while her hands were tied together. She was taken back to the Willagee house where she was gagged, chained to the bed and raped.

After Birnie had finished raping the girl, Catherine Birnie got into the bed with them. She now knew that this turned David on. When they had both assaulted her, Birnie tried to strangle the girl with the nylon cord, but she became hysterical. The Birnies forced sleeping pills down her throat to calm her down. Once Candy was asleep, David put the cord around her neck and told Catherine to prove her undying love for him by murdering the girl.

Catherine complied with the demand and killed Candy while David watched. When asked later why she did it, Catherine said: "Because I wanted to see how strong I was within my inner self. I didn't feel a thing. It was like I expected. I was prepared to follow him to the end of the earth and do anything to see that his desires were satisfied. She was a female. Females hurt and destroy males."

They buried Candy near the grave of Mary Neilson in the State Forest.[7]

On 1 November, they saw 31-year-old Noelene Patterson standing beside her car on the Canning Highway; she had run out of fuel while on her way home from her job as bar manager at the Nedlands Golf Club. Once inside the car, she had a knife held to her throat, was tied up and told not to move. She was taken back to Moorhouse Street where David repeatedly raped her after she was gagged and chained to the bed. The Birnies originally decided to murder her that same night, but David kept her prisoner in the house for three days and there were signs that he had developed an emotional attachment to Patterson. Catherine quickly became jealous and made an ultimatum: David would have to kill Patterson or Catherine would kill herself. He immediately forced an overdose of sleeping pills down Patterson's throat and strangled her while she slept. They took her body to the forest, but buried it away from the others. Catherine reportedly got great pleasure from throwing sand on Patterson's face.[8]

On 5 November, the Birnies abducted 21-year-old Denise Brown as she was waiting for a bus on Stirling Highway. She accepted a ride from them, and at knifepoint, was taken to the house in Willagee, chained to the bed and raped. The following afternoon she was taken to the Wanneroo pine plantation. In the seclusion of the forest, David Birnie raped Brown in the car while the couple waited for darkness. After they dragged Brown from the car, David Birnie raped her again and stabbed Brown in the neck. Convinced that the girl was dead, they dug a shallow grave and laid her body in it, but Brown sat up in the grave. David Birnie then grabbed an axe, struck her twice in the head, and buried her body in the grave.

Seventeen-year-old Kate Moir was abducted at knifepoint after accepting a ride from the Birnies.[9] Moir later stated that she asked them if they intended to kill or rape her, and was informed "we'll only rape you if you're good". She was forced to dance for them, and slept in the couple's bed while handcuffed to David.[10] Moir was their final abductee and the only victim to survive. After abducting her, David held a knife to her throat, and forced her to ring her mother. Moir assured her mother that she had too much to drink and was staying at a friend's house, hoping her mother would catch on the ruse and call the friend, knowing she was not a drinker.[11]

She escaped the day after her capture. After David went to work, Catherine went to the door to carry out a drug deal, and forgot to chain Moir to the bed. She escaped by climbing through a closed window by breaking its lock; however, she hit her head on the concrete.[11][12] After knocking on various neighbours' doors, she jumped a gate and was attacked by David’s dog. She managed to flee, and ran into a vacuum cleaner shop on 10 November 1986. She later described herself as "hysterical. I'm barefoot wearing my black leggings, a black singlet, no knickers...". She informed the shop owner that she had been raped.[11] When the police arrived, she said she had been abducted at knifepoint by a couple who had taken her back to their house and raped her. The police were initially sceptical of her story, but 22-year-old Constable Laura Handcock believed her from the outset, due to the amount of detail she provided, including their address and telephone number.[13] The Birnies had given themselves aliases, but Moir had read David's name on a medicine bottle.[14] Moir stated they had watched the film Rocky on VHS, and described a drawing she had concealed in the house as proof of her presence. Subsequently, the police found her drawing in the home, as well as the VHS copy of Rocky in the Birnie's VCR. David and Catherine were arrested, and during their interviews, they gave conflicting information; Catherine denied ever meeting Moir, while David insisted Moir had come to their house voluntarily to engage in consensual sex. Detective Sergeant Vince Katich used a hot soldering iron to convince David to confess, and reveal where they had buried the bodies so that they could be dug up before dark; David revealed there were four graves.[14]

There is speculation that the Birnies were responsible for the disappearance of Cheryl Renwick in May 1986,[15] and Barbara Western in June 1986.[16] It has been suggested that David was responsible for the disappearance of Lisa Marie Mott in 1980; however, his first wife accounted for his whereabouts on the day Mott disappeared.[17]

• Blundell, Nigel; Blackhall, Susan, eds. (2004). "David and Catherine Birnie". The Visual Encyclopedia of Serial Killers. London, England: PRC Limited. pp. 46–49. ISBN 978-1-85648-710-8.
• Westch, Elizabeth (2005). "Birnie, David & Catherine". Serial Killer Crime Index. Archived from the original on 27 September 2007. Retrieved 12 February 2007.
• Liebmann, Steve (30 October 2007). "The Moorhouse Horrors". Crime Investigation Australia. Series 1. Episode 6. Foxtel. Channel 9.