A woman is fighting for life after she was doused in petrol and set alight at a home in Melbourne's south east.

Mystery surrounds the circumstances leading up to the incident with police yet to charge anyone.

Emergency services found the injured 22-year-old woman when they were called to the Boronia Avenue, Cranbourne, property just after 6.30pm yesterday.

The woman was found about 6.30pm yesterday. (9NEWS)

Police investigations are ongoing. (9NEWS)

"We went inside and found a lady quite burnt," neighbour Daniel McCreedy told 9NEWS.

"It was quite distressing. We found a jerry can in the backyard on fire, which we put out, and then just proceeded to try and look after them."

The trashed backyard led to a bathroom inside where Mr McCreedy helped the woman's family extinguish the flames.

Daniel McCreedy spoke to 9NEWS today. (9NEWS)

"We were just trying to get her back into the shower. Cold water running, just to obviously ease the burns," he said.

She was later taken to Alfred Hospital in a serious condition and had skin grafts applied to her head, back and shoulders.

The woman remains in a critical condition, but her burns are so severe that the homicide squad has been prepared to take over the case.

They joined arson detectives scouring the residential street today, door knocking neighbours desperate for answers.

While the woman's estranged husband was assisting police with their inquiries but no charges have been laid.

It is believed the woman lived with her in-laws and a one-year-old child.

"They're a pretty quiet bunch, you never really hear any noises or hear any disturbances," a neighbour said.

Her brother-in-law sustained serious burns to his hand while trying to save her, he said.

Questioned by police for hours overnight, he was still wearing a seared shirt from the incident..

But told 9NEWS the woman was alone in the backyard when the fire started.

At least four children witnessed the event.

The investigation continues.