Firefighters pulled a man aged in his 30s from a burning, three-storey unit block in Hurstville early today after they were told no one remained in the building.

They found the man lying unconscious under a blanket on a lounge.

Emergency services were called to the building on Park Road shortly after 3am.

When Fire and Rescue NSW brigades arrived, flames and large amounts of smoke were pouring from a unit at the rear on the second level.

"One occupant - a man in his 20s - escaped before the fire took hold," a FRNSW statement said. "He was treated by NSW Ambulance paramedics for burns and smoke inhalation,"

The statement said about 20 people evacuated the building after being awakened by smoke alarms and police.

"While FRNSW crews had been told by residents that everyone had been accounted for, they carried out a search of the burning unit as a precaution," the statement said.

"Firefighters discovered a man, aged in his 30s, who was unconscious under a blanket on a lounge.

"He was carried downstairs and out of the building, before being taken to hospital in a serious condition.

"The blaze was contained to the unit and extinguished."

FRNSW Superintendent Adam Dewberry said working smoke alarms and an automatic fire door assisted with the rapid and safe evacuation of the unit complex.

"The situation would have been a lot more dangerous and complicated had the building not had those safety features and the flames were allowed to spread further," he said.

"This is a timely reminder to landlords that they are responsible for ensuring that smoke alarms are installed, working and less than 10 years old."

NSW Police, with the assistance of FRNSW's Fire Investigation and Research Unit, are investigating the cause of the fire.

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