A council ranger has been knocked unconscious and suffered serious facial injuries after what police believe was a dispute over parking in Sydney.

Police said the Liverpool City Council ranger was walking with another ranger along Norfolk Serviceway in Liverpool about 12.30pm on Tuesday when he was confronted by two people who got out of a white Subaru WRX Impreza.

Police would like to speak to this man following an attack on a Sydney council ranger. Credit:NSW Police

It's alleged one of the men punched the 35-year-old council ranger in the face, causing him to fall to the ground unconscious.

Witnesses intervened before the pair drove off and emergency services were alerted, police said.