A SOPHISTICATED hydroponic cannabis lab has been uncovered after a Sydney house fire.

Firefighters, who were called to a three-bedroom house on Merrylands Road, Greystanes, at 11.30pm on Friday, found a marijuana growing system throughout the house and a drying room for the crop.

No people were in the house but neighbours reported seeing a man leaving shortly before the fire started.

A police spokesman said a search of an adjacent garage, which was damaged in the blaze, led to the discovery of a second hydroponic system.

Police said at least 120 plants were located inside the property, with an estimated street value of about $500,000. The fire is believed to have been caused by an electrical fault.