Four men infected with Legionnaires' disease after spending time in Sydney CBD

Health authorities are investigating a possible link between parts of the Sydney CBD and Legionnaires' disease, after four men were diagnosed following visits to the Town Hall area.

The disease, which is a bacterial infection of the lungs, can be contracted by breathing in contaminated water vapour or dust, NSW Health said in a statement.

NSW Health and the City of Sydney have launched an investigation.

They are checking cooling towers, starting in the Town Hall area, because outbreaks can be linked to air conditioning systems in large buildings.

The sick men had spent time in the CBD during the period of their infection, including in the Druitt, Market, Sussex and Pitt Street areas, NSW Health said. 

They are all receiving treatment in hospital.

Legionnaires' causes fever, chills, a cough and shortness of breath, NSW Health Director for Communicable Diseases Dr Vicky Sheppeard said.

"The time from exposure to the onset of symptoms is typically between two and 10 days, so people who were exposed could have symptoms already or develop them over the next week," she said.

"The bacteria that cause Legionnaires' disease live in water and can multiply in the water used to cool air conditioning systems, so infection is prevented by routinely maintaining and treating these systems.

"People outside can be exposed to the bacteria when a water cooling system emits contaminated water particles into the air.

"People who develop this disease are diagnosed by chest X-ray and a urine test, and usually require antibiotic treatment in hospital."

NSW Health has urged anyone who lives, visits or works in the Town Hall area and who develops symptoms of Legionnaires disease to visit their GP.