A 22-year-old Gosnell man set his girlfriend on fire Tuesday afternoon. Police said the man, who is now in policy custody and has been charged with intent to cause grievous bodily harm, poured petrol on the woman and lit her.

The incident happened in a house in Perth. After the woman was set on fire, she was seen screaming through a park in Gosnells.

Steele St Clair-Baker, who initially heard the woman's screaming from nearby Tay Street, said he immediately called an ambulance after he saw the severity of the woman's burns. He then rushed to his house, got a bucket of water and held the bucket while the victim scooped water to douse her burns, thewest.com.au reports.

The woman, estimated to be in her early 20s, suffered serious burns on more than 40 per cent of her body, particularly the upper torso and face. She is still in critical condition at the Royal Perth Hospital.

The man also sustained minor burns on his hands and arms, and remains under police guard in the same hospital.

Mr St Clair-Baker described the man as more distraught than the injured woman.

The incident appears to have been triggered by a domestic dispute between the couple from New Zealand.