Duo impersonating police rob pair in Sydney's west

Police are appealing for information about a violent robbery committed by two men impersonating police in Sydney's west last week.

The pair allegedly approached a 19-year-old woman and a 21-year-old man who were in a Hyandi Elantra in Mt Druitt's Manse Reserve carpark on Thursday at about 2.40am.

Both men identified themselves as police officers before one of them smashed a window of the car with a baton.

They demanded the victims' money and other valuables before fleeing in a light-coloured Holden Commodore.

The men were described to police as being of Mediterranean or Middle Eastern appearance, 23 to 30 years old with lean builds.

In another incident, a 13-year-old girl, her grandmother and step-brother were approached by three men in a white-coloured Holden Commodore while walking on Station Street, St Marys on Saturday at about 7.20pm.

A man in the front passenger’s seat told the girl to come up to the car.

When the girl's grandmother questioned the man's motives, he claimed to be a police officer but could not produce his badge.

The group then activated a siren similar to that on a squad car and were last seen driving east on Phillip Street, St Mary's.

* Police urge anyone with information in relation to this incident to call Crime Stoppers on 1800 333 000 or use the Crime Stoppers online reporting page.