A Perth father who doused his two children in petrol and set one on fire has been found guilty of five charges, including two of attempting to kill the children.

Edward John Herbert pleaded not guilty to the charges on the grounds of insanity, but WA Supreme Court Justice Lindy Jenkins rejected that defence.

Herbert's three-year-old daughter suffered burns to her upper body and face in the attack, which the court heard caused severe and life-threatening injuries and left her with permanent scarring.

The second child, aged seven, was doused in petrol before being rescued by an off-duty police officer who had rushed to help from the house across the street.

During the trial, the court was told Herbert had been drinking and smoking cannabis on the day of the offences.

A doctor testified it was probable Herbert suffered from bipolar affective disorder and there was "evidence of psychosis" on the day and night of the incident that could be linked to his substance consumption.

Partner feared for her life, court told

In a statement tendered to the court, Herbert's partner said he was "getting angry all the time" and had "doubled his cannabis use" in the lead-up to the incident.

She said Herbert's anger escalated throughout the night of the offences and she began to fear for her life.

She said that around midnight he told her: "That's it bitch, I am going to kill you" before getting a jerry can of petrol and entering his daughters' room.

He poured petrol over the younger child and set her alight, before he began to douse the older child in petrol.

An off-duty police officer held back tears during the trial as she told the court how she rescued the children while Herbert watched on silently.

Another neighbour Daniel McMillan said he entered the home to restrain Herbert and asked him what he had done.

He told the court Herbert responded by saying he set his daughter on fire because she was "too beautiful", and "don't worry, I wouldn't have lit my boy up".

In a letter Herbert wrote to his partner from jail, he said he could not remember any of what happened on the night and he would have to live with it for the rest of his life.

"WTF happened?" Herbert wrote.

"I love my kids more than life itself ... they said I had a mental illness and smoking weed is very bad for my type."

Herbert sat emotionless as the judge delivered her verdict.

He is expected to be sentenced in May.