The naturally found Legionella pneumophila bacteria that causes the flu-like illness could spread as buildings prepare to re-open to the public following the coronavirus lockdown.

While there have been no known incidences of the disease this year in the Murrumbidgee Local Health District, cases have increased in Sydney compared with autumn in 2019.

NSW Health Senior Environmental Health Officer Tony Burns said the Legionnaires' bacteria, which is found in water, commonly spread to humans through water droplets released by air-conditioning systems.

"Most of us would have antibodies in our system because we've been exposed to it at some stage in our life. It's a problem if it gets into air-conditioning because we inhale the bacteria," Mr Burns said.

"The way air-conditioners operate, they have water that runs through them at times [and] droplets of water are broken down and put out into the environment and you could inhale a droplet of water that just contains the legionella bacteria."

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Mr Burns said air-conditioner maintenance could "fall down a little bit" over the cooler winter months after an already uncertain period where building owners may have been reluctant to engage contractors due to coronavirus fears.

"Building owners have got to employ a water treatment contractor and a maintenance contractor and those two have got to work together to ensure that maintenance is done regularly and the water treatment program is maintained," he said.

The bacteria causes a respiratory illness whose symptoms are "not unlike" the early stages of COVID-19, but which are usually easily treatable with antibiotics.

Legionnaires' disease poses a more serious threat in some severe cases or in the already vulnerable population.

"Legionellla bacteria is around all the time," Mr Burns said.

"Unless we control it we do get an increase in cases."