The Auckland Council has sent 15,000 letters to building owners, asking them to immediately shock-treat all cooling towers to prevent Legionnaires' disease.
In the past two months, there have been 12 reported cases of the illness in Auckland, one of them fatal.
The legionella bacteria that causes the disease is found in air conditioning cooling-towers.
Auckland Regional Public Health Service says all cooling-towers must be professionally treated with chemicals to combat the disease.
It says all businesses must contact the council urgently, so it can determine which towers have been tested and which have not.