Construction is under way for a 500-space three-storey car park on the northern side of the railway line due for completion in November.

The $13 million project includes 500 square metres of retail space, an undercover path between the car park and station, CCTV cameras, lift, bus interchange and kiss-and-ride drop-off point.

The project means 300 more car spaces in Harris Street and includes disabled car spots.

Penrith Council will decide how the retail space will be used.

State Londonderry MP Allan Shearan hailed the project as a big deal for the area and said it would encourage more workers to use public transport.

``There is a lot of demand in St Marys for more car parking,'' he said.

``It will make people's lives easier when they can pick up milk and bread here before heading home.

``We've found that shops traditionally keep the place safe.''

Construction started last week following an archeological dig on the historic site.

A hotel and stockyard stood at end of the railway line at St Marys before it was extended to Penrith in 1863.

Construction company AW Edwards has been appointed for the project.

Project manager Matt Bourne said several hundred tradespeople would be involved on-site.

St Marys is one of 10 car parks being built in western Sydney as part of the state government's commuter car park program.