Auckland District Health Board is reviewing its water system screening following an outbreak of Legionella at Starship Children's Hospital.
A child is still in intensive care at the hospital in a critical condition after contracting the disease last month.
Despite extensive testing at the hospital no other cases had been detected, according to chief medical officer Margaret Willsher.
"As a result of our investigations we have found that our water system was not functioning as intended."
Dr Willsher said Auckland DHB was reviewing its routine water screening.