She is understood to have serious burns to 40 per cent of her body, but was in a stable condition last night.
Mr St Claire-Baker held the bucket for the woman as she scooped water to pour over her burns. He said she appeared to be in shock but wasn't yelling or screaming.
The argument between the woman, who is in her 20s, and her boyfriend, Jonathan Pau'u, 22, is believed to have started at their home in the suburb of Gosnells.
Neighbours heard yelling before the pair spilled into the adjacent park.
They were also seen hugging in the park, before the woman was set alight and she started screaming for help.
Another neighbour told of seeing the woman being set alight.
"My neighbour has just set his girlfriend alight in Birrel Place," the witness said on Facebook. "Fire, cops and ambos are there now."
Emergency services staff called to the scene at first believed they were attending a house fire. Rescuers said they could smell petrol.
Mr St Claire-Baker said the boyfriend appeared even more distraught than the burned woman.
The couple were taken by ambulance to Royal Perth Hospital, with Pau'u under police guard.
He had burns to his arms and hands, and faced a court hearing yesterday on one charge of committing grievous bodily harm with intent to do so.
He was remanded in custody to appear in the Perth Magistrates Court next Wednesday.
