Auckland District Health Board says its water system was not working properly when a child contracted legionnaire's disease during a stay at Starship Children's Hospital last month.
The potentially fatal disease, which affects the lungs, usually comes from standing water.
Chief medical officer Margaret Wilsher says test results from water samples show that while the hospital meets water quality standards, it is still falling short of best practice for hospital environments.
Dr Wilsher says the board has already made some improvements but will also be making changes to its water distribution system, screening and maintenance.
She says the board is still confident the case was an isolated one.