Sydney police are investigating the theft of 12 cars from a holding yard in the city's south-west early this morning.

At around 6:00 am AEDT, a group of 13 men entered the Liverpool yard through the main gate and assaulted several workers, forcing them inside one of the cars.

They then stole some keys and took twelve cars, with a total value of more than $500,000.

Police have set up a crime scene and are canvassing the local area.