The victim, a 57-year-old Richmond man, had just left a function at a hotel on Swanston Street with a group of colleagues, when the attack occurred about 11.50pm on Wednesday, March 2.
Police said the attacker also left the hotel at that time and approached the group of work colleagues on the street.
The man allegedly punched the victim in the face, causing the 57-year-old to fall to the ground unconscious.
While the victim was lying on the ground, the man allegedly continued to assault him, and went on to assault two men who tried to get him away from the victim.
The alleged attacker then ran from the scene and the victim was taken to hospital with serious facial injuries.
Police have released photos and CCTV of a man they want to speak to about the attack.
He is believed to be of Caucasian appearance, aged around 30, with black shaved hair, facial stubble and sleeve tattoos on both arms.
He was wearing a black cap, black t-shirt with a CFMEU logo, khaki pants and white shoes.
Any witnesses or anyone with information is urged to contact Crime Stoppers on 1800 333 000 or submit an online report at www.crimestoppersvic.com.au.