NSW Health officials confirm that two water cooling towers tested positive for the bacteria that caused outbreaks of legionnaires' disease in the Sydney CBD earlier this year.  But the exact cause is still a mystery.

A man in his 80s died, and about a dozen other people were sickened in the outbreaks in March and May.  Legionnaires' bacteria is usually associated with air conditioning systems and hot water tanks.  But even hot tubs, humidifiers, and freshwater creeks and streams can host the potentially-deadly germs.

"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 to Fairfax Media.  "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."

The investigation by the panel set up by NSW Health recommends tougher regulatory measures to help prevent future outbreaks, including requiring owners to develop individual monitoring and control plans for each cooling tower system.  These changes could be made early next year.