A suspicious fire in Sydney's northwest is believed to have started in a bikie clubhouse before destroying two nearby factories.

More than 50 firefighters worked to contain the blaze that began in two units, understood to be used by an outlaw motorcycle gang, on Walker Street at South Windsor shortly before 2am today.

However, the blaze spread to nearby kitchen and cabinet-making factories.

Fire and Rescue spokesman Superintendent Chris Jurgeit said four units in an eight-unit industrial complex were destroyed, with firefighters battling "flammable liquids".

The cause is yet to be determined.

"It's in the hands of police and fire investigators," Superintendent Jurgeit said.