The 34-year-old was on Yarra Drive in Southbank at around 12.40pm on June 18 when five men he did not know rushed at him, police say.
The group allegedly threw the man to the ground and kicked him.
Police believe the group of assailants then drove to a location in Southbank and were involved in another assault.
They left the scene on foot, believed to have headed north on Clarendon Street towards the CBD at around 1.45am.
Meanwhile, the man was taken to hospital with serious but not life-threatening injuries.
Police released CCTV footage of five men they want to talk to about the attack.
Anyone who witnessed the incident or has information is asked to contact the police.