A woman found with horrific burns in bushland near Sydney is in an induced coma after a chemical attack that police believe took place a week ago.

After residents heard cries coming from bushland near Ferraro Crescent at West Hoxton on Friday night, police found Monika Chetty with burns to 80 per cent of her body.

The 39-year-old was rushed to hospital in a critical condition, with burns so severe that she cannot be physically identified.

Ms Chetty, who is in a critical condition, told police that she was approached by a man who threw a chemical on her about a week ago in a Liverpool park.

Police do not know how long she had been lying in the bushland, but they believe she sustained the burns before New Year's Eve.

Detective Superintendent James Johnson says Ms Chetty sought assistance from residents and was given food and water on December 31.

He says the woman was suffering from burns that are "black in nature and unmistakable" and is calling on witnesses to come forward.

"I don't believe this woman could have survived for the last week without having some assistance from members of the public," he said.

"We do know that on New Year's Eve she did seek assistance from residents and was given some food and water."

Resident Giola Motti says she decided to call police after seeing the homeless woman for several days.

"I've come out and she's at the top of the street lifting up a bag, throwing it over, dragging it through the bushland," she said.

"I thought, that's it, I'm done, call the cops, something's not right. And this is the outcome."

Police 'astounded' woman survived horrific injuries

Detective Superintendent Johnson says Ms Chetty was not able to supply any information about her attacker.

"This is the most unusual thing I have seen in my 41 years in the police," he said.

"I am astounded that anyone could survive this length of time with the horrific injuries that she has."

Detective Superintendent Johnson indicated that Ms Chetty was homeless and that she may have been living out of a green vehicle in the West Hoxton area.

"I don't think she was in the park that long," he said.

"There were a number of pieces of clothing hung up on the fence - we believe they belong to her.

"She is a woman who has no known place of abode and has been living in various locations in between the Liverpool and West Hoxton area."

Police say Ms Chetty has a husband and children which she has been estranged from for a number of years.

Police spoke to Ms Chetty's husband last night and are waiting to speak to him in more detail. He is not considered a suspect.

Officials are urging anybody with information to call Crime Stoppers on 1800 333 000.