05/05/2016 2:57 PM AEST | Updated 28/09/2016 9:59 PM AEST

The Second Outbreak Of Legionnaires' Disease In Sydney CBD This Year With 42 Cases Reported So Far

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SYDNEY, AUSTRALIA - MAY 05: Pedestrians walk towards Martin Place in the central business district on May 5, 2015 in Sydney, Australia. Forecasters are predicting the Reserve Bank of Australia will cut interest rates for the second time this year in its upcoming May meeting. The cuts would come amidst fear of stoking the rising prices of housing in Australia. (Photo by Mark Metcalfe/Getty Images)

A woman who contracted Legionnaires’ disease in Sydney’s CBD remains in a critical condition in hospital, as authorities test 70 cooling towers in a search for the source of the outbreak.

The woman, believed to be aged in her 30s, was admitted to Royal Prince Alfred Hospital after being diagnosed with the disease, which is a form of bacterial pneumonia.

Another woman, believed to aged in her 30s, is recovering in hospital and a man aged in his 80s is in a stable condition.

All three have been confirmed to have underlying medical conditions, with the first patient diagnosed on Friday last week and the other two incidents confirmed on Tuesday, according to health authorities.

The authorities are testing 70 cooling towers in the area bounded by Margaret, George, King and Kent Streets with one of those towers expected to be the source of the outbreak.

Director of NSW Health Communicable Diseases, Dr Vicky Sheppeard said testing was nearly complete but the results would not be known for a number of days.

"The public health units and the City of Sydney yesterday inspected 58 cooling towers in the region of interest," Sheppeard said.

Legionnaires’ disease -- a severe form of pneumonia caused by bacteria called Legionella -- occurs after inhaling contaminated water droplets that infect one type of cell in the lungs causing infection and in rare, extreme cases, death.

The disease can’t be contracted from another person or by drinking contaminated water.

This year alone there have been 42 cases of Legionnaires' disease across NSW, according to NSW Health, however this is only "slightly higher" than the average of 37 cases over the past five years.

"Given that we've had a couple of outbreaks, it's not surprising there's been an increase in the total number of cases," Sheppeard said.

Sheppeard said Legionella lives in water and can multiply in the water used to cool air conditioning systems.

“Infection is prevented by routinely maintaining and treating these systems," Sheppeard said.

The disease mainly affects middle-aged and elderly people and in particular those who smoke or have other medical conditions such as cancer, kidney failure and diabetes.

It’s the second outbreak this year to affect Sydney’s CBD. In March four men contracted the rare but potentially deadly disease around Sydney Town Hall by contaminated water, vapour or dust.

Sheppeard said the unusually high autumn temperature may be responsible for the outbreak, with the greater usage of cooling towers.

"Legionnaires' disease is a very serious disease, and it does have a mortality rate of 10 per cent," Sheppeard said.

Symptoms can occur within two to 10 days and include fever, chills, coughing and shortness of breath. NSW Health urge people to immediately contact their GP if symptoms occur.

A spokesperson from the City of Sydney said the City conduct a proactive risk based inspection and sampling program of approximately 1,400 cooling towers across its area.

“Under the Public Health Act, the City maintains a register of air conditioning towers and who is responsible for their maintenance."

The spokesperson said the City’s cooling system inspection program is up to date.

“Legionella is a notifiable disease. As public health is our number one concern, we are taking the matter very seriously, and providing every assistance to NSW Health who are leading the investigation,” the spokesperson said.

Health authorities and hospitals are watching out for any further cases of the disease, according to Sheppeard.

"Hospitals and public health units have been active with surveillance looking for any additional cases of Legionnaires' disease, as yet no further cases have been identified," she said.