A man has been left in a coma following a pitched brawl in Sydney's CBD involving around 30 people.

The 31-year-old man from Greenacre, in Sydney's southwest, was found lying on the ground at the corner of Sussex and Goulburn streets after the fight broke out between drinkers at the Star Hotel about 5.30am (AEST) today.

The man suffered serious head injuries and underwent surgery at Royal Prince Alfred Hospital, where he remains in a coma.

His condition this afternoon was reported as critical and his family were at his bedside.

Detective Inspector Peter Houlahan told reporters in Sydney that information on the attack was "scant" - the only CCTV footage available showed the victim already on the ground.