An elderly man is dead and another is recovering from legionnaire's disease in the small east Auckland community of Beachlands.
Health officials have confirmed the dead man and the sick man both lived in the same street.
Auckland regional public health service manager Dr William Rainger said the elderly mandied last month.
"We initially believed this to be an isolated case, as legionnaire's disease often is," he said in a statement.
"We investigated the household water supply as per our normal practice and found legionella bacteria.
"Legionella is a waterborne bacteria and there is no reticulated water supply in Beachlands. All of the households rely on roof collection for their water supplies."
The second case was a 40-year-old man, who was recovering at home. He lived across the street from the man who died.
He said the legionella bacteria was confirmed on March 17 in water supplies to the homes of the elderly man, his next door neighbour and the sick man across the street.
Manukau mayor Sir Barry Curtis said he was distressed to hear of the outbreak.
"I am assured that although legionella bacteria may be found in other roof-collected water systems, the risk this could cause illness appears small," he said in a statement.
Dr Rainger said legionella bacteria were also found in a waterblaster at the nearby Pine Harbour Marina.
He said health officers and the Institute of Environmental Science and Research had provided all three affected households with immediate and long-term treatment advice.
Local doctors, the Ministry of Health and the Beachlands community had also been advised how to protect their water tanks from pathogens such as legionella, giardia, cryptosporidium, and campylobacter, Dr Rainger said.
He said legionella had been found in roof-collected rain water supplies overseas, but never before in New Zealand.
Last year there were 11 confirmed cases of legionellosis in the Auckland area. All recovered.
