Ashley Mervyn Coulston was born on 10 October 1956 in Tangambalanga, Victoria, Australia. He grew up on his family's dairy farm and was described as a shy and secretive individual. At the age of 14, Coulston abducted two female teachers at gunpoint, forcing them to drive him interstate. The women managed to escape, and Coulston was subsequently sent to a youth training center. In 1988, he gained media attention by attempting to sail a self-built 2.4-meter boat, "G'Day 88," across the Tasman Sea, earning him the nickname "Captain Bathtub." He was rescued after 46 days at sea.

On 29 July 1992, Coulston responded to a newspaper advertisement placed by two university students, Kerryn Henstridge and Anne Smerdon, seeking a housemate for their Burwood residence in Melbourne. That evening, Coulston visited the house, where Peter Dempsey, Smerdon's brother-in-law, was also present. Armed with a sawn-off .22 rifle fitted with a homemade silencer and carrying cable ties, Coulston forced the three into separate rooms, hogtied them, and executed each with a single shot to the back of the head. He left the scene without leaving significant evidence.​

On 1 September 1992, Coulston attempted to abduct a couple, Richard and Anne Shalagin, near the National Gallery of Victoria on St Kilda Road. Using the same weapon and cable ties, he tried to restrain them, but Richard overpowered him, and the couple escaped. Coulston fired at pursuing security guards, injuring one, but was eventually apprehended by police. Ballistic tests linked his weapon to the Burwood murders.​

During his trial, Coulston remained mostly silent and showed no remorse. He was convicted of three counts of murder and multiple other charges, receiving three consecutive life sentences without the possibility of parole. He is currently incarcerated at HM Prison Barwon in Victoria.