David Birnie had some interesting sexual habits, the least of which was an excessive collection of brutal pornography. He was also known to use a hypodermic needle to inject localized anesthetics into his penis prior to engaging in intercourse. When the extreme measures he employed to achieve sexual excitement no longer worked, David suggested that it would be invigorating for Catherine to watch him rape someone. She believed him and relented.
Though the couple spent much time discussing murder in 1985, their first crime was not planned. Mary Frances Neilson was murdered after going to purchase tires from David Birnie. Mary was studying psychology at the University of Western Australia and worked part time at a deli. She had been planning her employ at the Community Welfare department as a counselor. Mary had been last seen leaving work for a lecture at the college on October 6, 1986. Six days later, her Galant sedan was found across the street from police headquarters in a parking lot where David Birnie had left it.
Following work that day, Mary had gone to David's workplace, but he asked her to meet him at home later to negotiate a price. Unable to resist the temptation, David forced her into the bedroom at knifepoint and stripped of her clothing. They bound, gagged, and chained her to the bed. Mary was raped repeatedly and, as Catherine looked on she asked her husband questions in the hope of discovering what she could do to excite him. Later, they took her to Glen Eagles National Park and raped her again. Mary begged for her life while David strangled her with a nylon cord, using a tree branch to gradually tighten the noose around her neck. He and Catherine mutilated Mary's body and buried it in a shallow grave.
This crime gave them encouragement, so they took out an ad in the local newspaper: "URGENT. Looking for a lonely young person. Prefer female 19 to24 years, share single room flat." It is not known if the ad brought any lookers, but the second victim did not find her way to the Birnies through this method.
Two weeks after the abduction of Mary Neilson, 15-year-old Susannah Candy was picked up while hitch-hiking on Sterling Highway. She had been an honors student at Hollywood High School. The couple held her for several days and raped her repeatedly. After David finished raping Susannah, Catherine would get into bed with them, knowing this would stimulate her husband. The Birnies made Susannah write two letters to her parents so they would not worry about her. She was to explain that she was safe and just wanted some time to sort out her problems. When they had finished having intercourse with each other on the last night, David attempted to strangle Susannah without success. He then stuffed her mouth full of sleeping pills and waited for her to fall asleep.
Then David told Catherine she could express her undying love for him by finishing off the murder. Catherine obeyed with enthusiasm, not letting go of the cord until Susannah was dead. When asked about this later, Catherine replied, "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 female. Females hurt and destroy males."
Noelene Patterson was on her way home from her job as a bar manager at Nedlands Golf Club who had been acquainted with the Birnies to some degree. The couple had helped to wallpaper the 31-year-old's home on a previous occasion. Noelene was having trouble with her car, and the Birnies came along to help her push it to a service station. Then they forced Noelene into their car at knifepoint and took her back to their home. They had planned to kill her the first night, but David kept putting it off. He raped her for three days before Catherine pointed a knife at her own chest and threatened to use it on herself. She forced David to choose between his favorite victim and his devoted wife. Catherine kicked sand in her face once she was dead and when Catherine took police to Noelene Patterson's grave, she spat on it.
Twenty-one year old computer operator Denise Brown was a social butterfly who spent most of her spare time dancing at clubs. She had just left the Coolbellup hotel and was headed home to the apartment she and her boyfriend shared with another couple when the Birnies abducted her at knifepoint on November 4, 1986. She had been waiting for a bus on Stirling Highway. She was raped for two days and then raped in the car while being taken to a pine plantation, where David stabbed her in the neck while raping her again. The stabbing failed to kill her and she sat up, screaming, while they buried her. David then hit her in the head with an axe. When she sat up again, he turned the axe head around and split her skull open with his final blow. Even Catherine was disturbed by the brutality of this final attack on Denise.
The afternoon after the couple had picked up Denise Brown, a 19 year-old girl was offered a ride by two people she later identified as the Birnies. She had been walking home from the university that day and remarked that there had been a third person in the back seat of the couple's car. She was upset to later discover that the third person in the car was probably Denise Brown, who would still have been alive.
The young woman said that something about the situation made her uneasy. The man was avoiding eye contact the whole time and the woman was downing rum and coke. She said the woman did all the talking. Their latest target had assumed the person in the back seat was probably the couple's daughter, but told the woman that walking was exercise she desperately needed and refused to get into the car. As she walked away, the couple continued to sit in their car a minute or two before pulling a U-turn and entering the pine plantation where they would rape and murder Denise Brown.