Steam from a cooling tower is suspected to be behind an outbreak of Legionnaires' disease in the Marlborough region.
Nelson Marlborough Public Health Service has tested 19 cooling towers and found one contained the same strain of bacteria which infected three people last month.
That is two more cases than are normally reported in the region for the entire year.
Regional medical officer of health Ed Kiddle says the three men worked in the same area as the infected tower.
Six more people who have symptoms of legionella are being tested for the disease.
Dr Kiddle says all the towers have been shock-dosed with chemicals to remove the bacteria and the threat seems to be over.