Just before 6.30pm yesterday, a man entered the self-service area at a supermarket on Patrick Street, Blacktown, where he was stopped by an employee after reportedly trying to leave without paying for items.
The customer allegedly pushed the employee, a 64-year-old man.
The employee fell backwards and struck his head on the floor.
The customer left the store and was last seen boarding a train at Schofields Railway Station.
The employee was treated at the scene before he was taken to hospital, where he remains in a critical condition.
Police are now seeking to speak to a man caught on CCTV footage who they believe may be able to help their investigation.
The man is described as being of Pacific Islander/Maori appearance, aged in his 20s to 30s, of a large build, with curly black hair and a beard.
On camera, he was wearing a black jacket, black Michael Jordan t-shirt, black pants and thongs.
Anyone with information is urged to contact Blacktown Police Area Command or Crime Stoppers 1800 333 000.