Three people have died in a suburban house fire, with investigators probing the cause of the blaze.

Police and fire crews were called to the home on Cornelia Street at Wiley Park in Sydney’s south-west, following reports of a fire about 9.45pm on Wednesday.

The blaze had reached the windows of the home by the time five fire trucks and 20 firefighters arrived, Fire and Rescue NSW said.

Crews forced their way through the property’s burning exterior extinguishing flames as they went, including on scaffolding and gardening trellis.

Three bodies were discovered in a granny flat at the rear, and are yet to been formally identified.

Fire and Rescue NSW Commissioner Jeremy Fewtrell attended the scene to offer support, and said investigations into the fire’s cause would be extensive.

“We offer our condolences to the families of the victims of this terrible tragedy,” he said.

“This is a stark reminder of the dangers of home fires, particularly during the winter period.”

Officers from Campsie Police Area Command have since established a crime scene and an investigation has begun, NSW Police said.

A report will be prepared for the coroner.

–AAP