A Perth teenager who killed his mother by slashing her throat may not have done so if he'd been given psychiatric help when it was sought, a trial has heard.

Gabriel Gaudi Lang was 19 and suffering a psychotic episode when he killed Joan Marie Hendry in their White Gum Valley home in December 2014, defence lawyer Tom Percy told the Supreme Court of Western Australia on Monday.

Perth lawyer Tom Percy is defending Gabriel Gaudi Lang. Credit:Facebook

Mr Percy said Ms Hendry had noticed her son behaving erratically and had taken him to the Alma Street Clinic in Fremantle seeking help, but left after waiting for more than two hours and having an encounter with a man who was talking about suicide.

"Tragically, had he been seen on that occasion, we might not be here today," Mr Percy said.