The two new cases of the pneumonia-like infection brings to five the the total number of patients linked to the recent outbreak, NSW Health said.

Both patients are men in their 40s and 50s and were in the CBD at a similar time as the first three patients.

The original three cases included a man in his 80s and two women in their 30s. One is in a critical condition, another is stable and the third has been discharged from hospital.

The bacterial lung infection causes fever, chills, a cough and shortness of breath, and can be fatal.

Dr Vicky Sheppeard, the director of the NSW Health Communicable Diseases Branch, said preliminary investigations identified five cooling towers that tested positive to legionella.

NSW Health declined to identify which specific buildings operated the towers.

"We've already taken regulatory action against those five towers and ordered cleaning and disinfection," Dr Sheppeard said. "We don't believe there is an ongoing risk in the city of Sydney."

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

It is understood that once NSW Health alerts the public to an outbreak, operators in the affected building often order the water towers to be cleaned before authorities arrive to test them.

Lax cleaning and maintenance by some building operators was one possible cause being investigated by the expert working group tasked with investigating the outbreaks, Dr Sheppeard said.

The group will consider beefing up regulations to protect the public from further outbreaks.

The original search area where health authorities were testing cooling towers for Legionella in the Sydney CBD.

"Potentially it can be related to the very long, warm period we've had that has required ongoing operation of the cooling towers but there may be other factors," she said.

Environmental health teams inspected all 89 towers and other potential sources in the area of common exposure.

The testing area stretches between Margaret, Kent, King and George streets, where all three patients had spent time before their symptoms began, as well as a number of buildings adjoining those streets, including buildings facing Martin Place.

"Contaminated cooling towers are a common source of legionnaires' disease outbreaks but they can often be contaminated without ever causing disease," Dr Sheppeard said.

"Further testing on these towers is being undertaken to determine whether or not one or more of these towers may be the source of the recent outbreak.

Legionnaires' disease occurs after a person breathes in contaminated water or dust, and outbreaks have been associated with the air-conditioning units used in large buildings.

Legionella bacteria get into the water supply that cools the system and multiply.

Passers-by can contract the disease from contaminated vapour drifting in the wind outside.

Those most at risk include middle-aged and older people, heavy smokers and those with chronic lung disease.

Legionnaires cannot be transmitted person-to-person.

"NSW Health and the City of Sydney have ensured appropriate regulatory action on these towers is under way, including orders requiring cleaning and disinfection," Dr Sheppeard said.

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.

NSW Health has urged the owners of CBD buildings to ensure their cooling towers are operated and maintained according to the requirements of the Public Health Act.

Under the Public Health Act 2010, building owners or occupiers that fail to maintain their cooling towers could be served with an improvement notice and ordered to carry out maintenance and repair.

If high levels of legionella bacteria are found, a prohibition notice may be served for immediate shutdown of cooling towers for emergency decontamination works and will not be allowed to operate until sampling results are received and a clearance certificate issued.

So far 47 legionnaires' disease cases have been identified in NSW in 2015, according to NSW Health data.

The state Labor Party has called on Health Minister Jillian Skinner to explain how the outbreaks have been able to flourish.

Opposition health spokesman Walt Secord has demanded the government set up a task force to ensure all appropriate steps are being taken to protect the public.

"The community must be confident that the government has taken all the necessary steps to protect the community," Mr Secord said.