Police want to hear from anyone who witnessed an assault outside a Spencer St bar at 1.45 am on March 24.
They say two men were involved in the incident and became aggressive after being refused re-entry to the premises.
One of the men punched one of the crowd controllers working for the venue, with the victim falling back and knocking his head against a wall.
The two men fled on foot and are believed to have moved south down Spencer St and then east along Flinders St.
One of the men is described as being of a slim build, in his early 20s and potentially of German background. He was wearing a blue cap, white shirt, a satchel bag and jeans. The other man was dressed in jeans and a blue jumper.
Anyone with information is urged to contact crime stoppers on 1800 333 000.