The hospital received confirmation from the Department of Health and Human Services on Monday that a patient had tested positive for legionella.

Legionnaires' disease is a serious and sometimes fatal form of pneumonia caused by the bacteria legionella. 

A St John of God Health Care spokesperson said there had been no death or fatality in this case.

The spokesperson said the department informed St John of God that the patient could have contracted the infection in a number of potential sites in Melbourne and in Bendigo. 

The spokesperson said the hospital used an "independent expert infection control specialist" and routinely monitored potential environmental risks. 

"We have been following all guidelines and complied fully with Department of Health and Human Services requirements," the spokesperson said.

"We are cooperating with the (department) while they complete their investigation to find the source of the legionella."

The department's website says legionella bacteria are found naturally in the environment and thrive in warm water and warm damp places. They are commonly found in bodies of water, soil and potting mix.

Legionnaires' disease is contracted by breathing in legionella bacteria in very fine droplets of water called aerosols.

Man-made water systems such as air conditioners sometimes provide environments that let legionella bacteria increase to large numbers.