Mayor Bill de Blasio said on Thursday the number of cases of the serious lung infection is up to 100.
Officials said earlier this week that the city’s largest ever Legionnaires’ outbreak, which centres on the South Bronx, had already peaked.
Investigators said the source was five cooling towers that have now been cleaned by the city’s health department.
Health officials are stressing that the disease, which is treatable with antibiotics, cannot be passed from person to person.
But many New Yorkers are taking precautions – switching from tap water to bottled water for fear of contamination.
There have been some 2,400 cases of Legionnaires’ disease across the US this year, according to the Centers for Disease Control and Prevention.
It can take up to 10 days for symptoms to appear after water infected with Legionella bacteria is inhaled into the lungs.
Sufferers can also experience fatigue, a loss of appetite, confusion, diarrhoea and headaches.
The airborne respiratory disease was named after a 1976 outbreak at an American Legion convention in Philadelphia.