The Auckland Public Health Service says two more cases of Legionnaires' disease have been reported this week.
In the past few months, there have been 15 cases of the illness in Auckland and one person has died.
The service says health officials are still trying to find out if the latest outbreak is connected to earlier cases.
The legionella bacteria that causes the illness is often found in air conditioning cooling towers, but the service says it is looking at other potential sources, including car washing systems.
Last week, the Auckland Council sent 15,000 letters to businesses asking them to immediately treat cooling towers with chemicals.