Queenstown residents can expect to boil their water for about four more weeks as the local council works to protect the water supply from protozoa.
There have been 72 confirmed cases in the local outbreak, caused by cryptosporidium. Another 20 are suspected and two possible are under investigation.
The boil water notice has been in place for some areas since 18 September.
Queenstown Lakes District Council (QLDC) is installing UV treatment at its Two Mile treatment plant, as well as bringing plants without the necessary protections up to compliance.
It will also clean the entire network supplied by the plant.
"This is a significant, one-off undertaking outside of normal business and something we've been planning alongside work at the treatment plant itself," QLDC general manager property and infrastructure Tony Avery said on in a release on Tuesday.
"It will include draining and cleaning Fernhill #1 Reservoir as well as cleaning and inspecting all other network reservoirs. We'll also be flushing the piped network to ensure any sediment where cryptosporidium could potentially still reside is displaced."
QLDC said Two Mile should be ready as planned on 8 December, with hopes water can be drunk straight from the tap the following week.
"Whilst the possibility of contamination during these activities is low and extreme care will be taken, they do introduce some additional risk," Avery said.
"It's still really important that people in affected areas keep boiling their water."
No one outside of the directly affected areas needed to boil water, Avery said. According to the QLDC website, the affected areas were Queenstown CBD, Fernhill, Sunshine Bay and Queenstown Hill.
"This continues to be a major project for council's infrastructure, legal and planning staff as well as contractors Fulton Hogan and Veolia. They are completing work in weeks that would typically take many months.
"We also continue to acknowledge the patience and resilience being shown by individuals and businesses in the affected areas."
The cause was established in early October as human faecal contamination.
RNZ
