At the Central Otago District Council meeting last week capital projects programme manager Patrick Keenan said both plants were non-compliant for protozoa treatment.
Protozoa barriers mitigated parasites such as cryptosporidium entering the water supply system.
The barriers were part of a nationwide strategy to prevent outbreaks of waterborne illnesses like the one that occurred in Queenstown in 2023.
Taumata Arowai required the barriers to be installed by the end of this year, Mr Keenan said.
It was estimated the Ranfurly installation — including design, construction, project management and commissioning would cost about $1.7million.
There was $2.5m budgeted in the 2024 annual plan and the draft 2025 long-term plan for the Ranfurly water supply, he said.
For Patearoa, which supplied about 50 urban properties and a rural water scheme of about 40 properties, the cost would be marginally less, at $1.6m and there was $1.7m in its budget.
Building a new water treatment plant at Patearoa was estimated to cost about $15m and for Ranfurly about $21m, Mr Keenan said.
A cartridge filter and ultraviolet protozoa barrier was recommended, as it was the most cost-effective.
The downside was people were likely to have to continue to restrict water use when the source water had high turbidity, or was dirty, due to rain.
The alternative of installing a membrane treatment system was discounted due to "very high" capital costs and higher operational costs, particularly electricity, Mr Keenan said.
A membrane system would also require the Ranfurly wastewater network to be extended to the water treatment site, at significant cost.
A tender for the work will be advertised this month.
Council Three Waters manager Julie Muir said there had been no problems with either of the existing plants.
"Nothing has changed.
"There is a difference between safety and compliance.
"There have been no complaints [about protozoa] ... there is no change to anything."
