Four men were admitted to hospital with Legionnaires' disease thought to have been contracted from air conditioning systems in central Sydney.
The men each spent time in the Sydney CBD, NSW Health said on Wednesday.
Cooling towers in the area are being inspected as outbreaks are often associated with contaminated air conditioning systems in large buildings.
Legionnaires' disease is a type of pneumonia caused by a bacterial infection of the lungs that can develop after someone breathes contaminated water vapour or dust.
NSW Health is urging anyone who has visited the Town Hall area and has symptoms such as fever, chills, a cough and shortness of breath to see their doctor.
Symptoms include fever, chills, a cough and shortness of breath, NSW Health's director of communicable diseases, Dr Vicky Sheppeard, told the ABC.
