Forensic police will this morning resume their investigations into a house fire that killed a four-year-old girl in Sydney's south west yesterday.

The girl's mother and twin sister managed to escape the blaze that took hold in their Claymore home yesterday afternoon.

Fire fighters tried to rescue the trapped girl but the intense heat forced them back.

Superintendent Rick Parkes from Fire and Rescue says forensic police and investigators will use sniffer dogs this morning to pinpoint the cause of the blaze.

"[It's] not just any sniffer dog, it's a highly trained dog that's used to determine if there's any hydrocarbons or any sort of accelerant present," he said.

"They gradually narrow it down from the building of origin to the area of origin, to the room of origin and then the object or specific location where the fire started."

Investigators say it is too soon to know what started the fire.

Superintendent Parkes says it is also unclear what fire systems were in the department of housing home.

"That's not determined at the moment," he said.

"That's something that fire investigators will be looking at, but because parts of the ceiling had come down that wasn't readily evident in the early stages of the investigation."

The girl's sister was discharged from Liverpool Hospital last night and her 22-year-old mother remains there in a stable condition.