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.

"He then retrieved a shovel from his car and smashed the bus windscreen before driving [away] from the scene."

The bus driver was uninjured.

A witness told Seven News the bus driver appeared to be "pretty shaken up".

The frenzied attack was caught on the camera phone of a passerby, and witnessed by several members of the public.

Officers from Marrickville Local Area Command are searching for the Subaru driver.

Police have the car's registration details and are confident they will be able to track down the man.

Police have urged him to come forward, and have described him as being aged in his early 40s, of Middle Eastern or Mediterranean appearance with black hair. He was wearing black pants and a blue shirt with white horizontal stripes at the time of the incident.

Officers were also reviewing the bus' CCTV footage.

"This behaviour is particularly concerning given it's on a public street," Inspector Natasha Fairfax said.

"It's unnecessary for people to react in that way. The bus driver is just trying to do his job," she said.

The damage to the bus cost roughly $1000 to repair.