Karl Lawrence Mentzer, 39, pleaded guilty in the Melbourne Magistrates' Court on Monday to stalking a woman he had dated from July 2024 to January. The court heard that Mentzer, who was addicted to cocaine, unleashed a campaign of harassment after the victim ended their secret relationship.
The pair had met through their professions but kept their relationship private as both were in other partnerships. Mentzer's stalking began in January when he traveled to a dog park near the woman's home in Sydney, where he knew she walked her dog, in an attempt to see her.
When the woman took a pre-planned trip to Tasmania in February, Mentzer was waiting for her at the airport arrival lounge. He approached her, stating, "I will ruin you, I will burn your life down." He later waited for her again at the Melbourne airport on April 15, despite her telling him she did not want to see him.
The harassment escalated after an hour-long phone call on May 3, during which the woman pleaded with Mentzer to leave her alone, expressing her fear of his escalating behavior. Despite her pleas, Mentzer continued to send her messages.
On May 7, the woman discovered Mentzer had attempted to book a room adjacent to hers at the QT Melbourne hotel. Although hotel staff moved her to a different floor, Mentzer installed a spy camera facing the hotel hallway from his room in an attempt to capture footage of her. Police later found hallway footage on Mentzer's phone. Throughout that night, Mentzer also sent her eight phone calls, 31 audio notes, and 30 text messages.
The following day, the woman saw Mentzer outside her Melbourne CBD office. When she tried to enter the building, Mentzer blocked the elevator doors and accused her of "financially crippling" him, threatening, "I'm meeting lawyers - you won't hear the last of me." The woman, visibly shaken, reported the incident to the police.
Investigators seized Mentzer's phone, discovering he had hired a private investigator to conduct surveillance on the woman, providing the investigator with her address, vehicle information, and photographs. The private investigator conducted surveillance at the woman's Sydney home on May 2. Mentzer's phone also contained motivational material about relationship break-ups, instructions for spy cameras, and secretly recorded footage of the woman and her husband.
Mentzer has prior convictions for assaulting a woman and has previously placed another woman in his car and driven away with her. His barrister stated that Mentzer abuses alcohol, ecstasy, cocaine, and dexamphetamine. Mentzer, the sole director of an IT company, has a close relationship with his mother, was sent to boarding school, is estranged from his father, and has limited contact with half-sisters.
Mentzer was remanded at Ravenhall Correctional Centre and is scheduled to appear in court again on July 30. Police had previously expressed concerns for the victim's safety, with one detective noting Mentzer's display of risk factors including a recent relationship end, sustained stalking, controlling behaviors, and severe jealousy, which are often present in lethal family violence incidents.