An after-school punch-up in the car park of an oval in Sydney's south on Monday left a boy knocked out cold on the concrete with serious facial injuries.

Sickening video footage has emerged of the fight, showing just how much more serious the incident could have been if the victim, apparently unconscious, had hit the back of his skull on the pavement instead of landing on his face.

Police said in a statement that officers attended the scene at Tonkin Oval, Cronulla, about 3.15pm on Monday "after hearing screaming", and were told the injured 16-year-old had been in a fight with a 15-year-old boy.

A spokesman for NSW Ambulance confirmed they had treated a teenage boy who, in his words, had been "king hit" in the car park of the oval, leaving him with left-side facial injuries.