People who have been in central Sydney are being warned to watch out for symptoms of legionnaires' disease after six cases were recorded in the past two weeks.

Two women and four men aged from their 40s to 70s visited the city on separate occasions in the 10 days before the onset of symptoms.

Legionnaires' symptoms can include fever, chills, a cough and shortness of breath, which can lead to severe chest infections such as pneumonia.

All six people were admitted to hospital with pneumonia, NSW Health said on Tuesday. One person has been discharged.

Five of the cases were reported on May 26.

The bacteria that causes legionnaires' disease is often associated with contaminated cooling towers of large buildings.

People can be exposed to the bacteria if they inhale contaminated water particles from the cooling system.

Environmental health officers have inspected and sampled 124 cooling towers to work out the cause behind the sudden rise in cases.

NSW Heath said legionnaires' disease could not be spread from person to person, but building owners must ensure their cooling towers were well maintained to prevent outbreaks.