Police are appealing for help to find four people involved in an acid attack on a motorbike rider in Victoria.

The 62-year-old man was riding along the South Gippsland Highway to Melbourne yesterday when he was flagged down, pushed down an embankment, and had acid thrown in his face.

He suffered serious burns to his face and body.

His attackers shouted that they "Didn't like bikies," but police say the victim is not associated with any outlaw motorcycle groups.

Detective Acting Sergeant Peter Johnston says he thinks it was a random attack and says police are scouring CCTV for the attackers' car, a small, dark green four-door sedan.