There is often a spike in cases toward the end of summer, NSW Health spokesman Dr Richard Broome said, but the increase in 2021 has been higher than normal.

Some 17 cases of the disease were notified across Sydney and the Illawarra in January.

The disease, which presents with symptoms similar to COVID-19, is caused by Legionella pneumophila bacteria often linked to contaminated air conditioning plants in large buildings.

"Public Health Unit staff investigate each case of Legionnaires' disease and no specific source has been identified," Dr Broome said.

"It's timely to remind businesses and building owners of their obligations under the Public Health Regulation 2012 to ensure their cooling towers are properly maintained."

The disease can cause severe chest infections like pneumonia and symptoms include fever, chills, a cough and shortness of breath.

It does not spread from person to person.

"Legionnaires' disease is diagnosed by chest x-ray and a urine test and usually requires antibiotic treatment in hospital," Dr Broome said.

"If you have tested negative for COVID-19 but have ongoing or worsening symptoms, you should see your doctor or visit your local emergency department."

Australian Associated Press