Man hurt during suspicious Sydney fire

A man is in a serious condition in hospital after cutting his arm while helping to evacuate units near a suspicious fire that destroyed a townhouse in Sydney's southwest.

Emergency services were called to Myall Road in Casula just before 1am on Friday to find the townhouse well alight.

More than 50 people were evacuated from adjoining units as Fire & Rescue NSW teams worked to extinguish the blaze which destroyed the townhouse.

Two elderly occupants were treated at the scene for smoke inhalation.

Police say a 30-year-old man badly cut his arm while attempting to help residents evacuate.

He was treated at the scene by ambulance paramedics and taken to Liverpool Hospital in a serious condition.

Police believe no one was in the townhouse at the time of the fire.

A crime scene has been established and will be examined by specialist forensic teams to determine the cause of the fire which police are treating as suspicious.