Emergency crews were called to the scene about 6.45am after reports of an explosion at the premises.

Firefighters said the Cooks Hill property caught alight in a blaze that forced crews to evacuate the unit complex.

It was not immediately clear how many people were evacuated from the structure.

Two apartments suffered extensive damage in the fire.

NSW Ambulance paramedics treated a man for extensive burns to his body that were believed to have compromised his airway.

He was subsequently flown to Sydney's Royal North Shore Hospital's burns unit in a critical condition for further treatment.

"These types of incidents are frightening, they can happen so quickly and the injuries can be catastrophic," NSW Ambulance Inspector Joel De Zuna said.

"As always our colleagues at Fire and Rescue NSW did an excellent job to create a safe environment for us to treat our patient.

"Severe burns and smoke related injuries can often be life threatening, it's vital that we start treating any type of burn immediately."

A Westpac Rescue Helicopter spokesperson the man had suffered burns to about 78 per cent of his body.

The man was placed on life support before being air-lifted to Sydney.

Police said an off-duty nurse treated the 41-year-old before paramedics arrived.

Six fire crews were on scene shortly after the report of the blaze, with Darby Street blocked off as they extinguished the inferno.

Police established an exclusion zone between Bull Street and Parry Street, urging motorists and commuters to avoid the area.

Darby Street has since reopened.

Fire investigators and police are investigating the source of the blaze.

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Fire and Rescue NSW Superintendent Scott Dodson said the blaze was a reminder that a fire escape plan could save a life. .

"Now is the time for residents to check they have a working smoke alarm and that all members of the family have an awareness of calling Triple Zero (000) in an emergency," he said.

The cause of the explosion was not clear at the time of publication.

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