Off-duty security guard charged with bashing a man inside a Sydney carpark

An off-duty security guard has been charged after allegedly seriously assaulting a man in a Sydney carpark.

He's accused of punching a man, aged 27, a number of times, knocking him unconscious, resulting in him falling and striking his head on the ground.

The alleged attack happened inside a carpark on Bond Street in the CBD last night.

NSW Police say the 27-year-old was approached by the security guard when he tried to open the door of a parked vehicle, just after 6pm.

The off-duty guard - who had just completed his shift - confronted the man before allegedly assaulting him.

The two men had been had been involved in a brief argument moments before the alleged attack.

The guard, aged 31, from Newtown, was charged with reckless grievous bodily harm and will front Central Local Court today.

The injured man was taken to St Vincent's Hospital where he has since undergone emergency surgery and remains in a critical but stable condition.