Queenstown could face months of having to boil water until treatment plants are upgraded with barriers against cryptosporidium, the mayor says.
Seventeen cases of infection from the parasite have been confirmed though RNZ has spoken to others who said they'd had stomach upsets but had not been to a doctor.
National water regulator Taumata Arowai has served a compliance order on Queenstown Lakes District Council for its Two Mile water treatment plant, which does not have a protozoa barrier to stop cryptosporidium entering the water supply.
A boil water notice must stay in place until it is upgraded or switched to another supply.
The area's other treatment plant, Kelvin Heights, had a protozoa barrier but there were doubts it was working, the regulator said. Once it was satisfied it was operating the boil water notice for the plant could be lifted.
"The source of the outbreak is still unconfirmed, but on the information available at the moment there is a material risk as it relates to drinking water," Taumata Arowai regulatory head Steve Taylor said.
Queenstown Lakes District Council mayor Glyn Lewers said five systems in the district did not have the filter systems in place. The council had been progressively upgrading and that work was being brought forward.
The council talked to suppliers on Tuesday and they would visit the site on Monday, when Lewers would have a better idea of when the equipment could be brought in and installed.
However Queenstown would face an extended period of having to boil water until the treatment plants were upgraded.
"I would say our best case would be months, at this point."
University of Otago parasitologist Bruce Russell said his hunch was the source was livestock waste.
There were often spikes in cryptosporidium cases throughout the country at lambing time, he said. "Infected animal waste is getting into water supplies and if you don't have a protozoa barrier there is a risk that water consumers can drink the infected spores."
The single cell spores produced by the cryptosporidium parasite are resistant to chlorine water treatment used against bacteria or viruses.
"They're got hard little shells that even resist our stomach acid, so they're very tough little customers.
"They need special treatment - usually UV and some sort of filtration."
Young children and babies were most vulnerable to gastro-enteritis from the parasite, so it was important that water used to make up milk formula was boiled.
Protozoa barriers and UV treatment equipment were expensive but it was unacceptable that the country had "developing world problem" with its water source.
"Clean drinking water is what should be one of the priorities of any developed and civilised society."
Action against other councils
Taumata Arowai regulatory head Steve Taylor said Queenstown District Council had to get on with its plans quickly.
"Regulations related to drinking water were introduced last year and those rules had a very clear requirement that water supplies like Queenstown Lakes District Council need to have a protozoa barrier so the fact one isn't in place is of significant concern to us," Taylor said.
The council was exploring treatment which could potentially be done "in the near term", he said.
"They need to get onto that very quickly."
There were a number of other water treatment facilities around the country which did not have protozoa barriers and it would be taking compliance action against them, similar to that in Queenstown.
Before the new rules came into effect in November 2022 the requirements on protozoa barriers were fairly loose, he said