A man has died after falling down the stairs of sex on premises venue "357" in Sydney’s Sussex Street it has been reported by SMH.

The 35-year-old man was leaving from the first-floor exit of Sydney City Steam on Sussex Street when he fell and hit his head, he was also carrying a bicycle, police said.

The venue reportedly did not know of the mans injury as a "passer by" phone an ambulance, not the venues staff. Paramedics found the man unconscious after a passer-by phoned emergency services about 1.50am, an ambulance spokesman said.

The man was taken to Royal Prince Alfred Hospital at Camperdown in a serious condition with head injuries, but he later died.

SMH reports that Sydney City Steam's manager would not say if the man was a colleague or a client, but said his "heart goes out" to the man’s family.