Hope footage will ID Vic CBD puncher

Investigators are hoping CCTV footage of a man will lead to a break in a case where a man was left unconscious after being punched in the back of the head in Melbourne's CBD.

The offender approached the 26-year-old victim, who was with his relatives on Bourke Street, and asked for a cigarette lighter at 3.30am on November 6.

The victim offered the man a drink of water and joked with him before walking away, only for the man to punch him in the back of the head, causing cuts and abrasions to the victim's face.

Police on Friday released CCTV footage of a man walking on a CBD street, who is then followed by a woman running after him, and are calling for the man to come forward.