Police have charged self-proclaimed mixed martial arts fighter Shaun McNeil with murder following the death of 18-year-old Daniel Christie on Saturday.

Mr McNeil had been charged with maliciously inflict grievous bodily harm and assault occasioning actual bodily harm following the alleged New Year's Eve assault but the charges were upgraded by police on Monday afternoon.

Daniel died on Saturday after spending 11 days on life support in St Vincent's Hospital with severe brain injuries.

Police allege Mr McNeil punched three underage boys on the corner of Victoria Street and Darlinghurst Road at Kings Cross around 9pm on New Year's Eve after the boys allegedly approached Mr McNeil and his girlfriend about buying drugs.