According to the FBI, a mass murderer is someone who kills four or more people with no cooling-off periods or breaks in between, contained to a single location. Mass murderers may be individuals or organisations: terrorist bombers and school shooters would fall under this category.

Spree killers will also have no cooling-off periods between their murders, but kill in more than one location. The usual example of a spree killer is Andrew Cunanan.

Location: Perth

Years active: 1986

This couple is responsible for what became known as the Moorhouse murders, named after the street where the killings took place. While they were never formally married to each other, David John Birnie and Catherine Margaret Birnie certainly had a twisted sort of connection.

The killer couple would bind their victims and rape them, sometimes keep them for a few days, before finally murdering them. Their final victim was Kate Moir, who thankfully managed to escape by breaking a window and running to nearby houses for help.

Location: Perth

Years active: 1959-1963

Known as “The Night Caller”, Eric Edgar Cooke started his criminal career as a vandal and escalated to arson, theft, and eventually, murder. The various ways that Eric Edgar Cooke killed his victims (hit-and-runs, strangulations, stabbings, shootings) made it difficult for police to realise that his crimes were related. In fact, two innocent people were initially convicted for some of his killings! Eventually, when he was caught, he gave detailed accounts of his crimes and put the picture together for the police.

Location: New South Wales

Years active: 1989-1999

What’s chilling about Kathleen Folbigg isn’t just the fact that she committed child murders – the children she killed were all her own! At first, all four babies were thought to have died of sudden infant death syndrome, but it was proven otherwise when Kathleen’s diary was discovered by her ex-husband. 

Kathleen Megan Folbigg grew up in foster care. She had become a ward of the state when her father, Thomas John Britton, stabbed her mother Kathleen Donovan 24 times, killing her. The morbid murder had been done in public, but Kathleen didn’t even know about this event until she was older.

Location: Queensland

Years active: 1998-1999

Leonard Fraser AKA the infamous Rockhampton Rapist was a serial killer and rapist who made headlines all over the country in the year 2000. What is most chilling about Fraser was that he was less than 2 years out of prison when he committed his most heinous crimes!

Fraser was jailed in the late 70s for the crime of rape. He then spent 20 years serving a prison sentence. Once he got out he almost immediately went back to his old ways and committed a slew of utterly disgusting crimes. He was officially jailed for the rape and murder of nine-year-old Keyra Steinhardt in 1999. And he was later found to have raped his terminally ill roommate, and murdered three other women between 1997 and 1999.

Location: New South Wales

Years active: 1947-1953

Also known as “Aunt Carrie”, or “Aunt Thally”, Caroline Grills was serial killer who used thallium to poison four members of her family! With her thick rimmed glasses and general grandmotherly appearance she certainly doesn’t fit the profile of a vicious killer. But make no mistake, Grills is as guilty as anyone on this list! 

Thallium is normally used as rat poison, but instead of using it on rats, Caroline Grills mixed it in tea and used it on her relatives. She never gave a clear reason for killing her family: she just killed them in cold blood.

Location: New South Wales

Years active: 1961-1962

William MacDonald went by many names throughout his life, but it was his moniker that’s the most unforgettable: the Mutilator. This is just a hint as to what William actually did with his victims – the details were so gruesome that some of the jurors fainted after hearing them!

When he was a teenager serving in the army, William was raped by one of his corporals. He claims that his killings were done in revenge for this rape – he was hearing voices in his head that the men he was killing were that corporal. Despite his insanity plea, William was found guilty of “the most barbaric case of murder and total disregard for human life” as described by the presiding Judge McLennan.

Ivan Milat: Known as the “Backpacker murderer”, Ivan Milat struck fear into the hearts of hitchhikers. He was convicted of 13 murders, but he’s suspected of murdering many more – he’s usually known as Australia’s most prolific serial killer.