A western Sydney businessman says he was struck by a car as he chased a robber who grabbed a bag containing $500,000 cash from his car moments after he arrived home.

The man, 45, said he parked outside his Lidcombe house and went to get the bag containing the day's takings from his business when another man took it and ran off about 9.45pm on Monday.

Police wouldn't confirm the figure, only saying it was a "large amount". The businessman told media it was $500,000.

The victim chased the robber for about 50 metres before he was struck from behind by a white sedan, police said in a statement on Tuesday.