Police have launched an investigation into the night-time assault, which happened about 11pm on Wednesday at Newtown, in Sydney's inner west.
Officers arrived to find a 25-year-old off-duty policeman unconscious on Enmore Rd. An off-duty nurse performed CPR until paramedics arrived.
The officer was taken to the nearby Royal Prince Alfred Hospital, where he is in a serious but stable condition.
Police were told the injured man and another off-duty officer were walking along the road, when they were approached by a group of three unknown men who were arguing with each other.
Both off-duty officers were assaulted and one was knocked to the ground, and repeatedly punched and kicked in the head, investigators said.
The three men ran from the scene before police arrived.
Investigators are looking for two men of Maori or Pacific Islander appearance, and a third man of Caucasian appearance, all between 20 and 30 years old.
Enmore Rd was designated as the first "special entertainment precinct" in NSW in late 2023, recognising the inner-city area's popularity as a late-night music and hospitality hub.
The designation allows extended trading hours for certain venues and looser noise controls for businesses.
30°C