Emergency services were called to a licensed premises on East Parade in Sutherland about 2am on Sunday.
Officers found a man, believed to be aged in his 30s, unconscious outside the hotel.
They performed CPR before the man was treated by paramedics but he died at the scene.
The man has not been formally identified.
Superintendent Craig Middleton said police were still determining the motive behind the alleged assault.
"The reasons for the altercation is still part of our investigation but I can confirm that the 31-year-old male had been asked to leave the premises previously," he told reporters on Sunday.
In an earlier release, police said they were told the security guard was allegedly punched to the head by a patron following an altercation.
Mr Middleton said CCTV footage and witness statements would be crucial to the investigation.
The 31-year-old man was charged with assault occasioning death.
He was bail refused and is set to appear before Sutherland Local Court on Monday.
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