Police have narrowed their search for a man who stabbed a restaurant worker at Southbank on Friday night.

Detectives are appealing to the public to help find Kenneth Scarlett, also known as "Kenny".

The victim of the attack had just knocked off work at a nearby eatery on the Yarra promenade in the city just before midnight on Friday when he saw a woman being assaulted by a man.

As he stepped in to help the woman, the offender produced a knife and stabbed him in the left side.