Emergency services were called at about 2pm today with multiple sections of the iconic building cleared as a precaution.
More than 15 firefighters remain at the scene trying to determine the source of the leak, while a number of people have been ordered to leave the area.
Initial reports suggested the leak occurred inside the carpark.
However, authorities are now working to determine if it may have started elsewhere.
Police are also at the scene while firefighters work to secure the area.
“We were called down this afternoon after reports about the smell of gas,” a spokesman for Fire and Rescue NSW told 9News.
“On arrival crews discovered that there were elevated levels of gas in parts of the building.
“We are currently waiting on the arrival of the gas company; we do have hazardous material crews who are going around monitoring the scene to ensure the gas stays at safe levels.”