A group of people who allegedly punched a Sydney man after he asked them to stop photographing his drunk friend may not be aware he had been critically injured, police say.

The man, from Eastwood, was helping his friend as he lay intoxicated on the footpath at Pitt St near the intersection of Goulburn St when the incident turned violent at 3am (AEDT) on Sunday.

Police say four men and a woman were taking photographs on their mobile phones when the 27-year-old approached them, asking them to stop.

They say an argument ensued and the man was punched in the face.

He managed to get up but was punched again and knocked unconscious as the group fled along Pitt St.

The injured man was taken to St Vincent's Hospital with critical head injuries and remained in a critical but stable condition with his fiance and family at his bedside.

"There's a chance these people ran off without knowing what really happened," City Central Commander Mark Walton told reporters on Monday.

"It's an example of how a fairly innocuous contact has evolved into a very, very serious injury."

Police have released CCTV footage of the group and are appealing for help in identifying two of the men involved.

The first man is described as being in his early 20s, about 183cm tall with a light complexion and a solid build.

He had short dark hair and was wearing a grey or blue round neck T-shirt, dark pants and dark shoes.

The second man is said to be aged in his late 20s, about 178cm tall, with a light complexion.

He had short dark hair and was wearing a black zip-up jacket, dark pants and white shoes.

Anyone with information about the attack should call Crime Stoppers on 1800 333 000.