Health authorities of Australia’s state of New South Wales (NSW) have issued a public health alert for Legionnaires’ disease in central Sydney. 

The NSW health department said that five people with recent cases of confirmed Legionnaires' disease all visited Sydney's CDB during their exposure period.

The alert said that the five patients developed symptoms between March 30 and April 4 after spending time in central Sydney, reports Xinhua news agency.

It said that “no single source” of the infection has been identified, and it is possible that the cases are “unrelated”, but that an investigation of a potential source area is underway.

Legionnaires' disease is a form of pneumonia caused by infection with Legionella bacteria, which is found naturally in freshwater. Symptoms typically develop between 2 and 10 days after exposure and can initially include headaches, fever and a mild cough.

According to the World Health Organisation (WHO), the mortality rate from the disease is usually within the range of 5-10 per cent but can be as high as 80 per cent for untreated immunosuppressed patients.

NSW Health said that the bacteria could contaminate air conditioning cooling towers, spas, shower heads, and other bodies of water.

“People walking outside or driving past may be exposed if they inhale aerosolised contaminated water,” it said.