A Perth father who set his 3-year-old daughter alight and doused her 7-year-old autistic sister with petrol while in a drug-induced psychosis has been sentenced to 17 years behind bars.

Edward John Herbert admitted intending to murder the girls at the family's Doubleview home in August 2015, but pleaded not guilty to five charges, claiming insanity. WA Supreme Court Justice Lindy Jenkins rejected that defence in April at a judge-alone trial.

Herbert had been drinking almost a carton of beer and smoking $50 worth of cannabis a day in the weeks leading up to the attack, in which he inflicted life-threatening burns to 13 per cent of his youngest daughter's body. She is permanently scarred.

Earlier that night, the 45-year-old shouted that aliens were coming to get him and said: "The werewolf is coming at 12 o'clock."

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He ran around naked and grabbed a knife, telling his now ex-partner he was going to kill her as she fled the house in fear.

Off-duty policewoman Stephanie Bochorsky heard a commotion from her nearby home and ran into the house.

She fought back tears as she told the trial she saw the 3-year-old standing in her cot with "her whole head on fire" and put out the flames using a blanket.

She then saw Herbert pouring petrol over the autistic girl as she lay in her bed and dragged the child out of the house while holding her burned sister.

The trial heard another neighbour rushed in with a fire extinguisher and found Herbert naked, pacing and drinking beer in the kitchen.

Edward John Herbert set his three-year-old daughter alight and doused her seven-year-old autistic sister with petrol while in a drug-induced psychosis. Photo / Facebook
Edward John Herbert set his three-year-old daughter alight and doused her seven-year-old autistic sister with petrol while in a drug-induced psychosis. Photo / Facebook

He told the neighbour he burned his daughter because she was "too f***ing beautiful", then lunged at the man with a knife but was hit on the head with the fire extinguisher.

When another neighbour rescued Herbert's 6-year-old son, the offender told her: "Don't worry, I wouldn't have lit me boy up."

Prosecutor Amanda Forrester said the boy had to be removed from his bed as he was frozen with fear from what he had heard.

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And if the policewoman hadn't intervened, it was highly likely Herbert would have set his older daughter alight, Forrester said.

Aside from disfigurement, the younger girl had suffered damaged vocal chords and hearing and would continue to feel pain from her burns, she said.

Justice Jenkins commended the neighbours for acting quickly and courageously, saying Bochorsky saved the girls' lives.

Trevor Hayden, a family friend, said outside court the 17-year sentence was not long enough for the heinous crime.

"No sentence can account for what that man done to those children," he said, according to the ABC.

"She's going to be scarred for the rest of her life. [It's] not enough," before saying Herbert could "rot in hell".

Herbert, who no longer suffers from psychosis or bipolar disorder, has been banned from contact with his daughters and ex-partner under lifetime restraining orders. An order applying to his son will expire two years after his release from jail.

Defence counsel Mara Barone said her client had told his psychiatrist he felt profound shame and guilt and was in "hell" knowing what he'd done to his daughter.

Justice Jenkins told Herbert his ex-partner had said he'd "broken her heart into many pieces".