St Marys' main street has been closed in both directions after a truck ploughed into the side of a building.
The incident occurred just after 2pm today, when the vehicle mounted the footpath near the intersection of Queen Street and King Street, with a witness telling The Daily Telegraph the truck was "carrying cars".
It's also been reported that the 54-year-old driver may have suffered a medical episode before the crash.
Emergency services responded to the incident, with four patients - including the driver - treated by paramedics at the scene, although none were taken to hospital.
The public is advised to avoid the area, as police, paramedics and fire rescue remain on the scene, and Live Traffic Sydney is urging motorists to consider an alternate route.