Surry Hills police are appealing for public help after a Rockdale man, 27, was found collapsed at Central railway station on Friday.
The man, suffering head injuries, entered the station near Chalmers Street at 7.30am before collapsing next to a stairwell.
He remains in a critical condition at St Vincents Hospital.
Police believe he was at Ward Park near Devonshire Street, Surry Hills, shortly before he walked to the station.
Investigators are still not sure how he was injured.
He is described as Mediterranean/Middle Eastern in appearance, mid-20s, 180-centimetres tall, of medium build with short dark hair.
He was wearing a South Sydney Rabbitohs jersey, black pants, blue shoes and carrying a dark shoulder bag.
Anyone with information about the incident should contact police immediately on Crimestoppers 1800 333 000.