The number of people diagnosed with Legionnaires Disease in Auckland in the past six weeks has risen.
The outbreak of the disease, a form of pneumonia, is believed to have claimed the life of an elderly woman.
Twelve other cases have now been confirmed.
Auckland Regional Public Health Service says only one or two instances of the disease would normally be reported in a six-week period.
No source has been identified, however the service is urging owners and managers of buildings with cooling towers that use water to urgently treat their air-conditioning systems with a suitable chemical to eliminate any Legionella bacteria present.
Legionnaires' Disease is often caught by breathing in mist from water or bagged potting mix and compost containing the bacteria.
In Canterbury, a strain found in potting mix is responsible for 15 cases this year.