Auckland District Health Board says it may not be able to find out how a child contracted legionnaires' disease during a stay at Starship Children's Hospital.
The potentially fatal disease affects the lungs and usually originates from standing water.
The child, who is already seriously ill with other health problems, is receiving intensive treatment at the hospital in Auckland after being diagnosed with the disease.
Child health director Richard Aickensaid the case is concerning, as many children at the hospital have suppressed immune systems and are vulnerable to infection.
Dr Aicken said taps, showers or air conditioning at the hospital may have been harbouring legionella bacteria.
He said the DHB hopes to get test results from water samples later this week, but there is a chance these will be inconclusive.
Dr Aicken said four other children in the same ward have been tested for the disease, but the results are negative and is very unlikely that any other recent patients or visitors have been infected.
It is the first case of legionnaires' disease to have been contracted by a patient while staying at Starship, he said.