Fynn, now 21, was hearing voices "screaming at him" to stab people and "get ready for war" and believed people were surveilling him and trying to steal his identity around the time he stabbed the couple in June last year.

Fynn had pleaded not guilty by reason of mental illness to four counts relating to the two stabbings and faced a one-day judge-alone trial before Judge John Hatzistergos last week that focused solely on his mental state around the time of the unprovoked attacks.

Judge Hatzistergos was shown CCTV footage from the shopping centre on June 18 that showed Fynn buying items from Coles before walking outside and confronting a man, 38, in the car park.

Fynn removed a 20-centimetre blade from his pants and suddenly started stabbing and slashing at the man, puncturing his left forearm. Fynn then sprinted after the man, who took refuge inside Kmart. Three days later the victim and his partner returned to Waratah Village to pick up his car when Fynn appeared and attacked again, this time repeatedly stabbing the woman, 26, in the arms and elbow.

On Monday, Judge Hatzistergos found that it was Fynn who had stabbed the pair, but noted both experts opined that around the time of the attacks Fynn was labouring under an untreated schizophrenic illness and had available to him a defence of mental impairment.

Psychiatrist Dr Richard Furst said Fynn was "suffering from paranoid delusions and hallucinations", including delusional beliefs that people were monitoring him and that his life was in danger. He also said Fynn was hearing voices and claimed one of the voices belonged to the man he had stabbed.

Judge Hatzistergos delivered a special verdict, finding the acts proven but Fynn not criminally responsible.

He was referred to the Mental Health Review Tribunal and will remain in custody.