A Perth father has been found guilty of setting his three-year-old daughter on fire and dousing her seven-year-old sister with petrol.

Edward John Herbert, who stood trial in the WA Supreme Court, admitted his intention to kill at the family's Doubleview home in August, but pleaded not guilty to five charges on account of being of unsound mind, according to news.com.au.

His trial heard last month evidence from a neighbour, Daniel McMillan, who went into the unit after the children were rescued and found Herbert pacing in the kitchen and drinking beer.

Herbert told his neighbour they were his children so he could do whatever he wanted and said his toddler was too beautiful and that is why he burnt her, reported Perth Now.

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When someone left the house with Herbet's son, he allegedly said "don't worry, I would not have lit my boy up".

Justice Lindy Jenkins, who heard the case without a jury, returned her verdict today and will sentence him on May 15.

Before sentencing Herbert's psychiatrist will prepare a mental health report.

The West Australian reported his lawyer Mara Barone conceded a lengthy jail term was now inevitable.