Queenstown and Frankton residents are being told to boil their water after an outbreak of illness from the parasite cryptosporidium.
Queenstown Lakes District Council said the notice affects all properties on the town water supply.
Residents and businesses should boil their water for at least a minute before using it for drinking, food preparation - including for baby formula, dishwashing, brushing teeth or for pets.
Eight people are known to have been infected with cryptosporidium, the council's infrastructure manager Tony Avery said.
Symptoms of sickness as a result of cryptosporidium could include diarrhoea, nausea and vomiting, cramps or pain, bloating, headache, lack of appetite, a light fever, tiredness or flatulence. And anyone with those symptoms was recommended to contact a doctor and not go to work or school, the government's HealthEd website said.
"The source of these cases is not yet known and there is no confirmed link to the local water supply," Avery said.
However, even after reports of illness in one of Queenstown's suburbs, the testing there did not include testing for cryptosporidium.
"On the basis of advice from [Health NZ] Southern regarding the nature of symptoms related to cryptosporidium infection and the potential speed and ease of transmission, we are issuing this notice," Avery said.
The town supply includes properties in Frankton, Quail Rise, Tucker Beach Road, Kelvin Heights and Hanley's Farm. Jack's Point was not affected as it was on a private supply, the council said.
Avery said in addition to the eight confirmed cases of cryptosporidium infection, there had been other reports of sickness in Fernhill, and extra monitoring had been carried out.
"We are aware of recent reports on social media of people living in Fernhill feeling unwell.
"All results to date have been, and continue to be, normal," but that testing did not include for cryptosporidium, Avery said.
Cryptosporidium is a tiny parasitic type of protozoa, and Fernhill's water supply does not have a "protozoa barrier" as part of the treatment process, he said.
Town water supplies have come under increasing scrutiny in recent years, after an outbreak of campylobacter in Havelock North where more than 8000 people are thought to have been infected, with four associated deaths and others left with long-term disabilities; as well as the revelations of high levels of lead contamination in some Otago water supplies, in 2021.
In July, it was revealed that three-quarters of the country's water supplies still did not have safety plans identifying potential hazards to water sources.
Details about cryptosporidium and symptoms can be found on the government's HealthEd website.
HealthEd said anyone who becomes sick with cryptosporidium should visit a doctor, stay away from school, early childhood centres or work, and avoid preparing food for other people.