Police have charged a man with four counts of murder following a fire at an aged care facility in Sydney's northwest.

The fire broke out at the facility on Hambledon Road, Quakers Hill, at about 5am local time (7am NZST) yesterday.

Three residents died in the blaze, while 32 others were taken to a number of Sydney hospitals.

Fifteen of them were admitted to the intensive care units at Royal North Shore and Concord Hospitals, and a fourth person died late on Friday afternoon.

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Following extensive inquiries by detectives, a man was arrested last night.

The 35-year-old registered nurse from Quakers Hill was charged early today with four counts of murder.

He was refused bail to face Parramatta Bail Court later today, although it could not be confirmed that he was employed by the old people's home.

Police had been treating the incident as suspicious after it emerged that the fire was believed to have broken out in two areas of the facility.

Superintendent Michael Willing, who is leading the case, said police acted on information they received after making an appeal.

"We received information from the public and that is being assessed as we speak," he told reporters.

The blaze broke out in the early hours of the morning after an automatic fire alarm went off with emergency services finding a "chaotic and tragic" scene.

Battling thick black smoke so dense they could not see their hands in front of their faces, firefighters evacuated around 100 disorientated and frail people, some blind or suffering dementia.

Willing said it had been a harrowing experience for everyone concerned.

"Everyone has been affected by this tragic set of circumstances," he said.

"This has had an impact on all the investigators but that pales into insignificance to what the families are going through. This has been a tragedy for Sydney and the entire community."

Investigators continue to examine the crime scene at Quakers Hill, and this is expected to take some days due to structural damage to the building.

They also continue to appeal for anyone with information about the incident to contact Crime Stoppers on 1800 333 000.

- AAP, AFP