A fourth case of potentially deadly Legionnaires' disease has been reported in Sydney.
A fresh alert has been issued by authorities after an increase in the number of people diagnosed earlier this week.
All four people in the cluster, including three cases confirmed yesterday, spent time in Clarence Street near Wynyard in the CBD between December 9 and December 18.
READ MORE: 'Driving me mad': Man claims recycling bin not emptied for months
Legionnaires' disease develops when Legionella bacteria gets into sources such as cooling towers on top of large buildings.
"People are exposed if contaminated water particles from a cooling system are in the air and then breathed in," Dr Vicky Sheppeard said.
"Symptoms can develop up to 10 days after exposure and include fever, chills, a cough and shortness of breath, and may lead to severe chest infections such as pneumonia.
READ MORE: 'Hundreds' expected to descend on new Bondi support hub
"Those most at risk include elderly people, people with underlying lung or other serious health conditions, and smokers."
All four people are in hospital, Sheppard said.
She added the source of the current outbreak is being investigated, with building managers in the CBD urged to check their cooling towers.
NSW Health said anyone who had been in the Clarence Street and Wynyard areas since December 9 should monitor for symptoms.
Infected people can develop symptoms between two and 10 days of initially contracting the disease.
DOWNLOAD THE 9NEWS APP: Stay across all the latest in breaking news, sport, politics and the weather via our news app and get notifications sent straight to your smartphone. Available on the Apple App Store and Google Play.