One unit was destroyed and another damaged by a fire in Whalan on Thursday night.
Emergency services were called to the scene on Aitape Crescent about 7.40pm.
Fire and Rescue NSW firefighters managed to control the fire.
The unit in which the blaze began was totally destroyed, while a neighbouring property sustained minor damage.
No one was inside the homes.
Mount Druitt police are investigating the cause of the fire.
The incident is being treated as suspicious.
Anyone with information should contact Mount Druitt police, 9675 0699, or Crime Stoppers, 1800 333 000.