Police are hunting for a Sydney personal trainer who has gone on the run since being sentenced to prison for an assault on a fellow dog walker in an inner city suburb.

David Mulligan repeatedly punched a man in the face when their dogs became entangled on a footpath in Alexandria on September 11, 2014.

He pleaded guilty to the assault that left his victim, Karl Nissen, with a fractured eye socket, a fractured jaw and a collapsed sinus that later led to a lung infection.

District Court judge Peter Maiden initially gave Mulligan a 15-month sentence but suspended it - meaning he could be released into the community - on the condition that he be of good behaviour and pay Mr Nissen $15,000.