A family of three has been killed in a devastating fire that engulfed a granny flat in Sydney’s southwest have been remembered as kind and devoted to their faith.

Investigators are now working to determine what sparked the deadly blaze.

WATCH THE VIDEO ABOVE: Three family members killed in Wiley Park granny flat fire

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The fire broke out just after 9.30pm at a property on Cornelia Street in Wiley Park, trapping a 52-year-old father, a 43-year-old mother and their 16-year-old son inside.

The victims’ family today described them as quiet people who kept to themselves and who were kind and devoted to their Buddhist faith.

All three were asleep when the flames took hold and were unable to escape.

“To lose three people in one fire like this is an absolute tragedy. It is devastating,” Commissioner Jeremy Fewtrell from Fire and Rescue NSW said.

The blaze was so fierce that firefighters struggled to reach the granny flat, which was located behind the main house at the end of a narrow driveway.

Flames seen pouring out of the home. Credit: 7NEWS

The destroyed home. Credit: 7NEWS

“Flames were pouring out of the windows and any other opening from the granny flat,” Superintendent Sheridan Waldau, Campsie Police Commander, said.

By the time emergency crews reached the property, it was already engulfed in flames.

The grandmother of the 16-year-old boy, who was in the front house, managed to escape to safety and was comforted by neighbours who rushed from their homes.

“Very nice people. But in the morning, when I heard about them, I feel very sad. My wife started crying,” witness Raj Ranaware said.

Firefighters putting out the blaze. Credit: 7NEWS

Campsie detectives are working alongside the State Crime Command’s Arson Squad and fire investigators to establish the circumstances surrounding the fire.

Firefighters have issued a clear message in the wake of the tragedy.

“Take the effort and the time just to check the smoke alarms. It will only take a minute or so,” Fewtrell said.