A Perth dad who set his daughter alight for 'being too beautiful' has been jailed for 17 years.

Warning: This article contains information about child abuse and violence which may be distressing for some readers.

A Perth father who set his three-year-old daughter alight and doused her seven-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, a defence that WA Supreme Court Justice Lindy Jenkins rejected in April after his 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 and left her 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.”

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 three-year-old standing up 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 burnt sister.

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

He told the neighbour he burnt 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 six-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 physically removed from his bed as he was frozen with fear from what he had heard.

And if the policewoman hadn't intervened, it was highly likely Herbert would have set his older daughter alight, Ms 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 Constable Bochorsky saved the girls' lives.

Herbert, who no longer suffers from psychosis or bi-polar disorder, has been banned from contact with his daughters and ex-partner under lifetime restraining orders, while 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".

If you or someone you know is in need of help, please call the National Sexual Assault, Domestic and Family Violence Counselling Service on 1800 RESPECT.