The bacteria was detected in two locations in the hospital, a shower in the hydrotherapy pool area and in another room.

So far there have been no confirmed infections.

The families of about 30 patients are being contacted by the hospital to warn them that their children may have been exposed to the bacteria in the past 10 days. The hospital says no other patients have been exposed or are at risk.

"There are no confirmed cases of infection, and the risk remains low. The Royal Children's Hospital has contacted all patients and families who have come into contact with the affected areas," a hospital spokeswoman said in a statement.

"As a precautionary measure, The Royal Children's Hospital has closed its hydrotherapy pool to allow the additional cleaning and testing to occur. The hydrotherapy pool, cooling towers, airconditioning and all other parts of the hospital are unaffected.

"Legionella is a common organism in the wider community, present in many water sources in the environment.

"There have been no reported cases of Legionnaires disease in people under 20 in Victoria.

"Those most at risk include adults on immune-suppressant drugs, people with chronic illness and smokers aged over 50 years of age."

Legionella bacteria can contaminate airconditioning units or spas. When inhaled they can cause a serious type of lung infection called Legionnaires' disease.

Early symptoms of the disease include coughing, shortness of breath, muscle aches, fever and headaches. Most people recover but the disease is occasionally fatal.