Boy punched in head on Melbourne tram

An appeal for public help has been launched after a teenager was assaulted by a man on a tram in Melbourne's north in May.

The attacker boarded the tram at Bundoora shopping centre with a man and two women around 4pm on May 29 before moving to the outside of the tram and tram surfed for several stops, police said.

When he re-entered the tram he approached a 16-year-old Thornbury boy and punched him in the head and upper body before two other men intervened and helped the boy depart.

Investigators have released a computer-generated image of a man they believe can assist with their inquiries and are urging anyone with information to contact Crime Stoppers on 1800 333 000.