The Auckland Regional Public Health Service says Legionnaires' disease has reached all four corners of the city and experts have no idea of the source.
An outbreak of the disease is believed to have claimed the life of an elderly woman. The heath service says her death was not unexpected, as she had other conditions and had been unwell.
Eleven other cases have been confirmed in Auckland in the past six weeks.
A strain found in potting mix and compost has led to 15 cases this year in Canterbury and seven deaths there since 2010.
Dr Julia Peters, a clinical director at the Auckland health service, says despite talking to all who have contracted the disease, no source has been found.
"The cases are spread right across the region - north, south, east and west. Some of the people have quite a small radius of activity, and I guess we can't rule out the possibility that there may be more than one source of the infection."
Building owners are being urged to treat air-conditioning systems with a suitable chemical that will kill any Legionella bacteria present.
Auckland Council's manager of building control, Ian McCormick, says cooling towers associated with industrial plants do not fall under the council regulation system and probably are not actively regulated at all.
Mr McCormick says there are many such industrial cooling systems in Auckland, and he would like to get as much information about them as possible.
Canterbury Medical Officer of Health Ramon Pink says gardeners should dampen potting mix, open bags carefully and wear gloves and a disposable face mask.
The disease and symptoms
Legionnaires' disease is a type of pneumonia often caught by breathing in mist from water or from potting mix and compost containing bacteria.
The bacteria does not spread from person to person and can be treated with antibiotics.
Symptoms include sudden high temperature and/or fever; a dry cough; loss of appetite; shortness of breath; chills, muscle aches and headaches; and sometimes stomach pain and diarrhoea.
