David John Birnie (16 February 1951 – 7 October 2005) and Catherine Margaret Birnie (born in 1951) were an Australian couple who were serial killers. They murdered four women ranging in age from 15 to 35 in their home in the 1980s, and attempted to murder a fifth. These crimes were referred to in the press as the Moorhouse murders, after the Birnies' address at 3 Moorhouse Street in Willagee, a working class suburb of Perth, Australia.

David Birnie was the eldest of five children. In his formative years, he lived in the semi-rural suburb of Wattle Grove, east of Perth. School friends and parishioners from the Wattle Grove Baptist Church of the period remember the family as particularly dysfunctional; rumours abounded about the family's promiscuity, alcoholism and that they engaged in incest.

In the early 1960s, his parents decided to move the family to another Perth suburb, where he had met Catherine through mutual friends. At 15, David left school to become an apprentice jockey for Eric Parnham at a nearby Ascot race course. During his time there he often physically harmed the horses and developed the tendencies of an exhibitionist. On one particular night, David broke into an elderly lady's house naked with stockings over his head and committed his first rape.

By the time he was an adolescent, he had been convicted of several crimes and had spent time in and out of jail for misdemeanors and felonies. As an adult, he was a known sex and pornography addict, and paraphiliac. He was married to his first wife during his early 20's and had a baby daughter.

In late 1986, David Birnie was employed at a local car wreckers. For more than a year David and Catherine had practiced how to make their sexual fantasies of rape and murder come true; he was weeks away from committing his first horrific crime.

Catherine Birnie (nee Harrison) was also born in 1951. She was 2 years old when her mother, Doreen, died giving birth to her brother, who died two days later; unable to cope with her, her father, Harold, had sent her away to live with her maternal grandparents. At the age of ten, there was a custody dispute where Catherine's father gained sole custody of Catherine again.

At the age of 12, she met David Birnie, and by the age of 14 she was in a relationship with David. Harold had begged Catherine on several occasions to leave David due to the fact that she was getting in trouble with the local police all the time. But the disapproval of their relationship only strengthened their union.

Her time in prison throughout her adolescent years offered Catherine the chance to break away from David Birnie. Encouraged by a parole officer, Catherine began working for the McLaughlin family as a house keeper. She married Donald McLaughlin on her 21st birthday.

She and McLaughlin had seven children; their firstborn, a son, was struck and killed by a car in infancy.

Four weeks after the birth of her seventh child, she abandoned McLaughlin and began cohabiting with Birnie, who had tracked her down in hospital after she had had a hysterectomy. She had her surname legally changed by deed poll to match his, and reportedly was emotionally dependent on him.

On October 6th, 1986, 22-year-old student Mary Neilson turned up at the Birnie house to buy some car tyres. She had approached Birnie at his work at the spare parts yard and he had suggested that she call by his house for a better bargain. As Neilson entered the Birnie house, she was seized at knife point, bound and gagged and chained to the bed. Catherine Birnie watched as her lover repeatedly raped the girl. She asked him questions about what turned him on the most; this way she would know that Mary Neilson would eventually have to die. They took her to the Gleneagles National Park where David Birnie raped her again before strangling her with a nylon cord and stabbing her through the heart; she was then buried in a shallow grave.

The second murder on October 20th when they abducted 15-year-old Susannah Candy as she walked along the Stirling Highway in Claremont. Within seconds of being in the car, she had a knife at her throat and her hands were bound. She was taken back to the Birnie house, where she was forced to send letters to her family saying that she had run away to Queensland with her friends before being gagged, chained to the bed and raped. After David Birnie had finished raping her, Catherine Birnie got into the bed with them, and David Birnie tried to strangle the girl with the nylon cord, but she became hysterical and went berserk. The Birnies forced sleeping pills down her throat to calm her down, and once Susannah was asleep, David put a nylon cord around her neck and Catherine tightened the cord slowly until she stopped breathing. They buried Susannah Candy in another shallow grave in the State Forest.

On November 1st, they saw 31-year-old Noelene Patterson standing beside her car on the Canning Highway; she had run out of petrol while on her way home from her job as bar manager at the Nedlands Golf Club. Once inside in 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 Birnie repeatedly raped her after she was gagged and chained to the bed. They had originally decided to murder Noelene Patterson that same night but David Birnie kept her prisoner in the house for three days and there were signs that he had developed emotional feelings for Noelene Patterson. Quick to notice, a jealous Catherine made an ultimatum, David would have to kill Noelene or she would kill herself. He immediately forced an overdose of sleeping pills down her throat and strangled her while she slept. They took her body to the forest and buried it along with the others. Catherine Birnie reportedly got great pleasure in throwing sand in Patterson's face.

On November 5th, they abducted 21-year-old Denise Brown as she was waiting for a bus on Stirling Highway. She accepted a lift from the Birnies; at knife point, Denise was taken to the house in Willagee, chained to the bed and raped. The following afternoon she was taken to the Wanneroo pine plantation. Safely in the seclusion of the forest, David Birnie raped Denise Brown in the car while the couple waited for darkness. As they dragged the woman from the car, David Birnie assaulted her again and plunged a knife into Denise's neck while he was raping her. Convinced that the girl was dead, they dug a shallow grave and lay her body in it, but Brown sat up in the grave; David Birnie then grabbed an axe and struck her twice at full force on the skull with it before burying her body in the grave.

Their final victim, and the only victim to survive their attacks, was seventeen-year-old Kate Moir. She ran semi-naked and ran into a vaccum cleaner store on 10 November 1986 and insisted on seeing the police. When the police arrived, she alleged that she had been abducted at knife point by a couple who had taken her back to their house and chained her to a bed, and that the man had repeatedly raped her while the woman observed. The next morning, while the man was at work, the woman unchained her and forced her to telephone her parents to say she had spent the night at a friend's house and was okay. The woman then led her back to the bedroom, but left to answer the door before securing her; the girl then escaped out the window. She told the police the phone number and address of the couple who had abducted her.

When the girl and the police arrived at the Birnies' residence, Catherine Birnie admitted that she recognized the girl but refused to answer any more questions without her husband. When the police brought David Burnie home in handcuffs, the couple claimed that the girl had not been abducted, but had willingly come to the house to share a bong with the Birnies, and that all sexual activity had been consensual.

The Birnies were detained by police, who tried to trick them into confessing to the crimes by intense interrogation. Around dusk, Detective Sergeant Vince Katich said in a joking manner to David Birnie, "It's getting dark. Best we take the shovel and dig them up." Birnie replied, "Okay. There are four of them." The Birnies were reportedly very excited, even proud, to show the police the locations of the graves of their four victims.

When sent to trial, David Birnie pleaded guilty to four counts of murder and one count each of abduction and rape. When asked why he pleaded guilty, he gestured toward the victims' families and said, "It's the least I could do." He was sentenced to four consecutive sentences of life imprisonment. After being found sane enough to stand trial, Catherine Birnie was also sentenced to four consecutive sentences of life imprisonment by the Supreme Court of Western Australia.

Initially David Birnie was held at the maximum security Fremantle prison, but he was soon moved to solitary confinement to keep him from coming to harm from other prisoners. The original death row cells were converted for him and he stayed there until the prison was closed in 1990. The cell can now be viewed on the Great Escape Tour held daily at Fremantle prison. While incarcerated, the Birnies exchanged more than 2,600 letters but were not allowed any other form of contact.

David Birnie was found dead in his cell at Casuarina Prison on 7 October 2005. He had committed suicide by hanging; he was due to appear in court for the rape of a fellow prisoner the next day.

Catherine Birnie is imprisoned in Bandyup Women's Prison, where she is the head librarian. Her first application for parole in 2007 was rejected, and the then Attorney-General of Western Australia, Jim McGinty, said that her release was unlikely while he remained in office.

Her case was to be reviewed again in 2010; however, on March 14th, 2009, new Western Australian Attorney-General Christian Porter revoked Catherine Birnie's non-parole period, making her the second Australian woman to have her papers marked "never to be released".