18,000 letters are going out to businesses in Auckland today as the council tries to combat the spread of Legionnaires' disease.
The number of people infected with the disease stands at 12, including one who's died.
Auckland Council building control manager Ian McCormack says businesses with cooling towers need to treat them with a large dose of chemicals to kill any Legionella bacteria.
"And we're particularly interested in any commercial application for cooling towers where they're part of process cooling in a manufacturing set up."
Mr McCormack says the towers are all being dosed as a precaution.
"Because at this stage there's no evidence that it definitely is one of ours. All we do know is the strain of Legionella we're dealing with, it lives in water."
He says history tells us cooling towers are one of the common ways Legionella bacteria gets into the air.
Mr McCormack says people should call 09 353 9358 if they have a cooling tower and to confirm it's been treated.