NSW Health has issued a warning following reports that three people have been confirmed with Legionnaires' disease.
The three cases were caused by Legionella pneumophila bacteria, which is a bacterial infection of the lungs.
Each of the people had spend time in Castle Hill, in Sydney's north-west.
Legionnaires' disease cannot be spread person to person, so NSW Health has said that the Hills Shire council and public health units are currently inspecting the cooling towers of Castle Hill for contamination.
Outbreaks can also relate to contaminated air conditioning systems in buildings.
Legionnaires' disease causes chills, a cough, fever and shortness of breath, and the time between exposure to displaying symptoms is usually between two and 10 days.
"People who develop this disease are diagnosed by chest xray and a urine test, and usually require antibiotic treatment in hospital," NSW Health's Communicable Diseases director Dr Vicky Sheppeard told the Daily Telegraph.