Residents in Melbourne's north-east are being warned of a legionnaires' outbreak that has already put three people in hospital.

Two men, aged 72 and 56, and a 38-year-old woman were treated at the Austin Hospital after contracting the potentially deadly bacterial infection, which authorities believe is linked to air conditioning cooling towers.

Victoria's chief health officer, Dr Rosemary Lester, said the three cases were detected in the past month.

Two of the patients live and work in Bundoora, and the third patient in an adjoining suburb.

Dr Lester said samples were taken from a local cooling tower as the state health department investigated the cause of the illness.

"The cooling tower was sampled on Friday and shut down until it was disinfected today as a precaution," she said in a statement today. "It will continue to be monitored, but the test results are not expected for several days."

Dr Lester urged any Bundoora resident with flu-like symptoms to be tested for the disease.

Legionnaires' disease causes flu-like symptoms such as headache, fever, chills, muscle aches and pains, followed by respiratory problems and pneumonia.

Dr Lester said most people affected were elderly, as is usually the case with legionnaires' disease, though it was more serious if people smoked or had a pre-existing respiratory illness.

Legionnaires' disease is contracted through breathing in very fine droplets of water containing bacteria, such as spray drifts vented off from cooling towers.

Decontamination and cleaning infected towers should eliminate the risk, the Victorian health department said.