St George-Illawarra's NRL premiership celebrations have been overshadowed by the death of a man who was arrested after a brawl outside the St George Leagues Club overnight.

Police were called to a fight outside the club in Kogarah at about 12:30am (AEDT), after a security guard was attacked.

They say they arrested two men, one of whom was tasered, and the other was targeted with capsicum spray.

The second man, a 35-year-old, was taken to hospital after he lost consciousness and has since died.

Police say the exact cause of his death is not yet known.

A witness at the scene, who gave her name as Kristy, said the man was repeatedly tasered while in handcuffs.

"There was three people, they capsicum sprayed the guy that was really messed up," she said.

"From that, he was handcuffed laying down on the ground, and they just kept tasing [sic] him.

"He asked them to stop, they kept tasing him, and then they started restarting his heart, he stopped obviously breathing."

Police have formed a critical incident team to investigate the death and say they cannot make any further comment.