The number of confirmed cryptosporidium cases in Queenstown has risen to 21, an increase of 4 in the last 24 hours.
Dr Michael Butchard, Medical Officer of Health, said this afternoon the numbers were current as at 10am today.
A source of infection had still not been identified, he said, but investigations were ongoing.
The news comes after Queenstown Lakes District Mayor Glyn Lewers apologised for the town's drinking water crisis, which has residents facing months of having to boil water and could cost $30 million to fix.
"I want to pass on my sympathies to the community and businesses," Mr Lewers said at a press conference this morning.
"We did not want to be in the situation, but you deal with the cards you’re given."
National water regulator Taumata Arowai has served a compliance order on the 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.
Mr Lewers said this morning a permanent solution could cost up to $30 million, while a temporary solution over the next few weeks could cost "hundreds of thousands to millions".
"We will have to defer non-core projects, go back to basics," he said.
Lewers said the council was opting for UV treatment for the Two Mile treatment plant as it was the quickest to install.
But it would not elimimate the risk of boil water notices in some situations, such as a decrease in water clarity.
The more effective protozoa barrier was more expensive but was more effective as a permanent solution. Council staff estimated the cost at $30 million.
"I have instructed the team I want a plan by the end of next week with timings so I know exactly what is going on and when it will be done, and that will be communicated to the community," he said.
The council was investigating tankers, Lewers said.
He said his assessment of months of boil water notices was a "worst case scenario".
Earlier today Mr 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.
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.
'Now is a particularly bad time'
Rees Hotel chief executive Mark Rose said the lack of water treatment filters was a "terrible" situation.
"We don’t know when this [Cryptosporidium outbreak] started and we’ve got no idea when it will end.
"The reason we don’t know those two things is because the water has not been filtered."
Council media spokesman Sam White said timeframes and costs for further investment in protozoa barriers would be included in the council’s long-term plan process.
Mr Rose said he had a number of cancelled bookings to his 155-room lakefront facility this week.
It was never a good time to be issued a boil water notice, but with New Zealand and Australian school holidays beginning next week and a Chinese holiday at the end of the month, "now is a particularly bad time".
"The reputational damage [for Queenstown] ... would probably be quite large."
A person with knowledge of the town’s water system, who did not wish to be named, said the council’s water testing was inadequate.
It was not doing enough testing at different points of the water distribution process or getting results quick enough to be aware of risks.
Council infrastructure operations manager Simon Mason countered the claims, and said the supply was monitored regularly, in accordance with last year’s drinking water quality assurance rules.
"Unfortunately, Cryptosporidium is not specifically identified by any of the routine testing undertaken, and nor is it a contaminant that can be tested for in ‘real time’."
The person claimed the council’s "single-source" Lake Wakatipu network was an issue because it meant water supplies could not be substituted and potentially contaminated water could not be isolated.
Mr Mason said "as a result of investment currently under way", water would soon be brought into the Frankton area from the new Shotover Country borefield and treatment plant.
— RNZ and Matt Porter