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 at Farmborough Heights in 2013.

Quinn, who is a quadriplegic from his self-inflicted wounds, maintained during his trial that he had long wanted to end their relationship and Ms Vize was accidentally wounded during a struggle when she tried to stop him from stabbing himself in the chest

His defence team argued that even if his actions were found to be deliberate, Quinn may have been so substantially impaired at the time that his charge could be reduced from murder to manslaughter.

The Crown argued that Quinn was in a jealous rage after learning about Ms Vize's relationship with another man and was determined to kill her and then himself.

Handing down his judgment in the NSW Supreme Court on Thursday, Justice Robert Beech-Jones said he was satisfied beyond reasonable doubt that Quinn had formed an intention to kill Ms Vize and then himself.

Justice Beech-Jones said the injuries to Ms Vize's neck and right hand were inconsistent with her intervening to stop Quinn from harming himself.

He also said that he found Quinn's claim that he wanted to end their relationship was "implausible" and  incompatible with "the overwhelming weight of other evidence" and he had been "untruthful" and "unreliable" in the witness stand. 

"I conclude that any aspect of the accused's evidence can not be independently confirmed...or corroborated by other evidence should be rejected," Justice Beech-Jones.

While accepting that Quinn suffered from an abnormality of mind, namely obsessive compulsive disorder and borderline personality disorder, Justice Beech-Jones found that the accused's capacity to control himself was not substantially impaired so his charge should not be reduced to manslaughter.

Quinn, he said, had demonstrated an element of planning in placing a knife in his pocket, cutting Ms Vize's throat and neck and then turning the knife of on himself.

Quinn and Ms Vize, who both studied art, had an on-off relationship for about three years. But two different portraits of their romance was painted during the trial.

Giving evidence, Quinn claimed Ms Vize was needy and unstable and said he had wanted to break up with her for some time but did not want to cause her any distress.

He said he thought his suicide would "free her up to be with another guy".

Ms Vize's mother and best friend said Quinn was obsessive about Ms Vize, calling her repeatedly and turning up at her home uninvited, and he could not be upset that she was happy in a new relationship.

In the 11 days before her death, Quinn contacted Ms Vize's mobile phone 227 times.

Ms Vize's mother and father and friends wept in the courtroom after the verdict was delivered.

Quinn is due to be sentenced on October 14.