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Health authorities have identified two water cooling towers that harboured the specific bacteria strain responsible for the two legionnaires' disease outbreaks in the Sydney CBD.
But the ultimate source of the outbreaks can't be proven beyond doubt, an investigation has concluded.
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The outbreaks served as a warning for water cooling tower operators who did not comply with the state's strict cleaning regulations, NSW Health Director of Health Protection Dr Jeremy McAnulty said.
A total of 15 people contracted the pneumonia-like condition after they visited the CBD during two separate outbreaks in March and May. One patient, a man in his 80s, died after contracting the infection.
Thirteen patients tested positive for the same legionella strain identified in two water cooling towers, one on Clarence Street and the other on York Street, according to the joint investigation conducted by NSW Health and the City of Sydney.
"While we cannot pinpoint the ultimate source of the outbreak, we were able to confine the origin of the bacteria to buildings in downtown Sydney west of Hyde Park between Park and Margaret Streets," NSW Chief Health Officer Dr Kerry Chant said.
"Proving the exact source of the outbreak is often difficult as cooling towers may be cleaned even before they can be examined," Dr Chant said.
"The aim of a public health response is to stop the outbreak as soon as possible by ensuring that cooling towers and other possible sources of infection in a location suspected to be the cause of the outbreak are controlled as quickly as possible," she said.
The other two patients diagnosed around the same time, including the man who died, had different strains and appeared unrelated to the outbreaks, Dr Chant said.
Further regulatory action against the Clarence Street site is being considered, and the York Street premises received a warning letter from the City of Sydney, the report read.
Twelve water cooling towers tested positive for legionella levels above the regulatory threshold in March - more than ten legionella colony forming units per millimeter of water (cfu/mL). Six towers also tested above 10 cfu/mL in May.
Two towers had excessively high colony counts (over 1000 cfu/mL). The Clarence Street site returned a result of 9800 cfu/mL and was issued a prohibition order.
Failing to comply with the order carries a maximum penalty of $55,000 and/or up to 12 months' imprisonment for an individual and a $275,000 fine for a corporation.
"NSW public health regulation is very strict … all towers should be free of legionella, or as low as possible," Dr McAnulty said.
"Where we found excess levels there was immediate action taken either verbally, or sending an improvement notice or prohibition order [to the cooling tower operators]," he said.
He said the death of a patient following an outbreak was "heartbreaking".
"Certainly this should be a wake up call: if you have a water cooling tower you need to make sure you are operating in a safe way," Dr McAnulty said.
The City of Sydney Council has roughly 1200 registered cooling towers within the CBD's 12.3 kilometre radius. On any day 610,000 people visit the area in addition to the resident population, according to the report.
"Legionella outbreaks are very different to, say a food-borne outbreak where we can ask the people affected where and what they ate. We might be able to work out, for instance, that it was the potato salad," Dr McAnulty said.
"But we can't be that specified in this case. We can asked people where they travelled to find commonalities, but we have hundreds of cooling towers in vicinity that might have been the source but it's really difficult to pinpoint.
"It's very frustrating from a public health point of view. We would love to say it's this one or that one, but our aim is to try and make sure all cooling towers are clear of legionella," he said.
The investigation drew on physical inspections of all cooling towers in the vicinity, the city's water cooling register, and satellite imagery to identify any potential unregistered cooling towers.
The affected patients who contracted the disease were interviewed and tested for specific strains of legionella, and for the first time whole genome sequencing was used to confirm the common strain in five people.
Labor health spokesperson Walt Secord said the report showed the NSW government was unprepared to manage outbreaks as temperatures rise.
He called for a comprehensive plan which included a significant increase in state government monitoring and inspection of air cooling tower systems.
An expert panel convened in May delivered several recommendations to reduce the risk of future outbreaks, including risk management plans for operating cooling towers. The panel did not recommend increasing penalties for tower operators who failed to comply.
The state government supports the proposed changes in principle but plans to consult further, Dr McAnulty said.
Any regulatory changes are expected to be made in early 2017, he said.