About 500 commuters were evacuated from Sydney's Town Hall station on Monday morning after smoke billowing from an engine room triggered an alarm.

A spokeswoman for Fire and Rescue NSW said the first call came through at 9.33am, and the CBD station was evacuated five minutes later.

Investigators at the scene confirmed smoke was billowing from an engine plant room due to an overheated mechanical bearing, the spokeswoman said.

No further resources were sent to the scene, but the station remained evacuated at 9.50am.