A case of road rage turned violent when a man used a shovel to smash the windscreen of a bus in Sydney's inner west on Monday.

The bus driver of the metrobus M30 service had dropped off his last passenger on Marrickville Road just after 9.30am before letting a car overtake him.

The gesture appeared to have upset the driver of a Subaru station wagon travelling behind the bus, police said.

"The driver approached the driver's window and verbally abused the [bus] driver," police said.