The South Eastern Sydney local health district is advising anyone who has been in the Sydney CBD and Clarence St area since December 9 to be aware of signs of the severe respiratory illness.
The warning comes as three people, unknown to each other, developed Legionnaires' disease after spending time near those areas between December 9 and 18.
The source of the outbreak is not yet confirmed and it is possible the cases are not linked, the local health district's acting director of the public health unit Professor Mark Ferson said on Sunday.
Symptoms can develop up to 10 days after exposure and include fever, chills, a cough and shortness of breath and can lead to severe chest infections such as pneumonia, he said in a statement.
"Those most at risk include elderly people, people with underlying lung or other serious health conditions, and smokers."
Outbreaks sometimes occur when bacteria from environmental sources including soil or cooling towers atop large buildings become contaminated.
NSW Health says it is working closely with the City of Sydney to test cooling towers in the area.
Legionnaires' disease is caused by infection with Legionella bacteria.
It is not spread from person to person.
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