A man who claimed he accidentally stabbed his on-off lover to death when she tried to stop him from taking his own life has been found guilty of murder.

Michael James Quinn, 27, fatally stabbed Cherie Vize, 25, in the neck before turning the knife on himself on the front lawn of his family home in Wollongong in 2013.

The Crown’s case was that Quinn, who is a quadriplegic from his self-inflicted wounds, was in a jealous rage when he stabbed Ms Vize after discovering she had started a new relationship.

Handing down his judgment in the NSW Supreme Court on Thursday, Justice Robert Beech-Jones said Quinn’s claims that he was not “upset in the slightest” about the break-up and never intended to kill Ms Vize were “completely implausible” and “untruthful”.

Quinn claimed he had felt trapped in the tumultuous relationship and wanted to end it, but decided to kill himself to avoid hurting Ms Vize and to “free” her to be with another man.

When Ms Vize tried to wrest the knife from him, according to Quinn’s version, her throat was accidently slashed and he then severed his spine in order to “punish myself and feel pain”.

His defence team argued that even if Quinn had deliberately killed Ms Vize, his criminal culpability should be reduced from murder to manslaughter because he was substantially impaired by an abnormality of mind at the time.

Justice Beech-Jones said the evidence suggested Quinn had “brooded” over the possibility of killing himself and Ms Vize for at least some days and that he was in control of his actions when he stabbed her to death.

While Quinn had been diagnosed with obsessive-compulsive disorder and borderline personality disorder, Justice Beech-Jones said he had planned the murder to some extent by seizing a knife, selecting the neck as the place to stab Ms Vize and then immediately attempting to take his own life.

Justice Beech-Jones said the wounds suffered by Ms Vize, including defensive style wounds to her hand, were inconsistent with her having intervened to stop Quinn from self-harming. Quinn had, he concluded, had the intention to carry out a murder-suicide.

“[Quinn’s] completely implausible evidence concerning the crucial events of the last few weeks of Ms Vize’s life compel me to reject any aspect of the accused’s evidence that was not confirmed or corroborated by independent evidence,” he said.

Friends and family of Ms Vize, who was studying art and spent much of her spare time going to art shows, tutoring and looking after her relatives, wept as the judgment was read.

Ms Vize’s mother and best friend gave evidence during the trial about Quinn’s obsessive behaviour, including calling Mz Vize repeatedly, showing up at her home uninvited and threatening self-harm if she left him.

Quinn contacted Ms Vize’s mobile 227 times in the 11 days before her death, while she had only contacted his nine times during the same period.

Quinn, who is in custody in a hospital facility in Long Bay jail, is due to be sentenced on October 14.