REPUBLIKA.CO.ID, PERTH -- A Perth man who doused his two young daughters in petrol and set one of them alight has been sentenced to 17 years jail.

Edward John Herbert, 45, was found guilty of attempting to murder the children at their Doubleview home in August 2015, when they were aged three and seven.

The three-year-old girl was in her cot when he set fire to her, causing life-threatening burns to 13 per cent of her body, including her head and chest.

The Supreme Court was told she is permanently scarred and now has problems with her vocal chords and her hearing.

The seven-year-old, who has autism, was dragged from the room by a neighbour, an off-duty police officer, who testified at Herbert's trial that when she entered the bedroom the three-year-old's "whole head was on fire".

A third child, a six-year-old boy, escaped unharmed, with Herbert telling witnesses later he would not have "lit up my boy", and that he burnt his daughter because "she was too beautiful".

Herbert was also convicted of threatening to kill his partner and trying to stab a male neighbour who had gone to the house after hearing the disturbance and managed to subdue Herbert by hitting him over the head with a fire extinguisher.

'If he didn't know, he should have'

Herbert claimed he could not be held criminally responsible for his actions because was psychotic at the time, but he was found guilty of the charges after Justice Lindy Jenkins ruled Herbert's voluntary use of cannabis and alcohol had contributed to his condition.

The court heard Herbert, who had been diagnosed with bipolar disorder, was regularly and heavily using cannabis and drinking beer in the weeks leading up to his crimes, and the change in his behaviour was noticed by his partner.

In a letter to the court, Herbert said he had "no idea, his use of cannabis and alcohol would affect his mental state".

But that was not accepted by Justice Lindy Jenkins, who said Herbert should have been aware from his history, which included a raft of offences, that his use of the substances would have a deleterious effect on his mental health.

"If he didn't know, he really should have known the risk he was taking," she said.

'Burn in hell'

Family friend Trevor Hayden said outside court the sentence was not long enough.

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

"She's going to be scarred for the rest of her life... [It's] not enough."

He said the child was still getting treatment, and until she was older she would have to keep having operations.

"Burn in, rot in hell," he said, referring to Herbert.

Herbert will have to serve 15 years before he can be released on parole.

He is also subject to a restraining order stopping him from contacting his former partner and his children.

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