Three people have been diagnosed with Legionnaires' disease since Anzac Day in the second outbreak to affect Sydney's CBD this year.

NSW Health and the City of Sydney are inspecting 67 cooling towers in the area bounded by Margaret, Kent, King and George streets, where all three patients had spent time before their symptoms began.

The patients - an elderly man and two younger women, all of whom had underlying health conditions - were all in the area between April 25 and 28.

One was expected to be discharged from hospital today, while another is stable and the third remains critical.

The bacterial lung infection occurs - most often in smokers, the elderly or people with depressed immune systems - after inhaling contaminated water vapour or dust and can be associated with the air conditioning units used to cool large buildings.

NSW Communicable Diseases Branch director Vicky Sheppeard said building owners should ensure that their air conditioning systems were properly monitored and disinfected.

"There's a couple of potential reasons why we're seeing more of these cases at the moment," Dr Sheppeard said.

"It has remained warm so during this time when we would [not] normally see more Legionella cases there may have been more use of cooling towers. Also we have raised awareness among medical practitioners and the public to test for this condition so we may just be picking up more cases than normal."

Nine people were hospitalised and a man in his 80s died as a result of the outbreak in the CBD in early March, while three were hospitalised in an outbreak in Kogarah later that month.

The last investigation, which focused on cooling towers and other potential sources of contamination in most of the blocks between Bathurst, King, Elizabeth and George streets, identified Legionella in 13 cooling towers, Dr Sheppeard said.

Twelve of them have been ruled out as the source of the infection and health authorities are waiting for the results of further analysis on the final one.

But it is possible that the source will never be identified because the contaminated tower was cleaned before it was inspected.

"We understand that with the warnings we sent out last time, even though we had field staff there the very next day, building owners were aware and quickly got on and cleaned their towers to make sure they weren't creating illness," Dr Sheppeard said.

The March outbreak was considered separate to the current one.

"Obviously we're keeping an open mind throughout the investigation to see if there are any links but it's been a very long break between the March outbreak and these cases which started on Anzac Day."

City of Sydney health and building manager Peter Harding said a team of 16 inspectors were inspecting the towers and the council could order building owners to clean their towers if a low level of Legionella was identified or decommission the tower if high amounts were present.

Health authorities and the council will re-test any buildings that were contaminated or not properly maintained in March during the current round of inspections.