The Greenough family massacre was the axe murders of Karen MacKenzie (31) and her three children, Daniel (16), Amara (7), and Katrina (5), at their remote rural property in Greenough, Western Australia, on 21 February 1993. They were killed by farm hand William Patrick Mitchell, an acquaintance of MacKenzie. Details of the murders were withheld from the public as they were considered too horrific. The case led to calls for the reintroduction of the death penalty.

Mitchell had spent the Sunday evening before the attacks getting high on a mixture of cannabis, alcohol, and amphetamines. The following Monday morning at around 3:00 a.m., sixteen-year-old Daniel, who was still awake, heard the sound of a car approaching the MacKenzie home. His sisters and mother (aged five, seven, and thirty-one, respectively) were all believed to be sleeping at the time. Mitchell parked the vehicle, exited and began walking towards the home. Curious as to who could be visiting at such an unusual hour, Daniel switched a kitchen light on, which Mitchell noted from outside. Daniel walked outside and attempted to greet Mitchell; not receiving any response, nor realising that the man was brandishing a tomahawk-style axe, Mitchell murdered him with the axe. The teen was the first victim discovered by family friends, face-down on the dirt drive.

Mitchell then entered the house, where he found MacKenzie asleep in the lounge room. He began attacking her in a similar fashion, killing her with multiple blows from the axe. He then went to the bathroom and retrieved a tube of lotion and a plastic bag to place over MacKenzie's head, after which he raped her corpse. Amara and Katrina were still likely asleep in their bedrooms when Mitchell murdered them. It was suggested that the younger two children were sleeping when they were killed, and that their lives ended relatively quickly.

Police and forensic investigators scoured the murder scene and collected evidence. A search party fanned-out in an attempt to locate any evidence or means of identifying those responsible, an effort which lasted seven days. In the meantime, the funeral for the four victims, which was attended by Mitchell, was held on 5 March 1993. It would be a further five weeks before Mitchell, an acquaintance of MacKenzie, was arrested on either 28 or 29 March.

Mitchell pleaded guilty to four counts of wilful murder and four counts of sexual assault. A hand lotion used by the killer was a key piece of evidence. Mitchell was convicted of the murders in 1995 at the age of 24 and was sentenced to four concurrent terms of life imprisonment, with a non-parole period of 20 years. According to the television series Crime Investigation Australia, a judge ruled that the exact way in which Daniel, Amara, and Katrina were killed was to be sealed.

Mitchell is currently incarcerated in Bunbury Regional Prison in Western Australia. Due to a public outcry against the sentence, a Crown appeal ordered the non-parole period to be revoked. There followed a series of Supreme and High Court appeals, including a ruling that Mitchell would never be released. An appeal overturned the non-release ruling and reinstated his 20-year non-parole period, and he consequently became eligible for parole in 2013, with a three-year review in 2016. In September 2013, Mitchell was refused parole. Attorney General Michael Mischin stated that his decision to refuse parole was based upon the gravity of the crime and the safety of the community. He became eligible for parole again in October 2016, and was refused parole again. As required by statute, his next review by the board was due in September 2019. In 2018 due to new McGowan government law which delays parole consideration for mass murderers and serial killers for a period of six years, Mitchell was no longer eligible for parole in 2019.

• List of massacres in Australia
• Axe murder