One of Queenstown's treatment plants will get its pipes flushed on Wednesday as the town tries to clean up its water supply.
Parts of the top resort spot have been under a boil-water notice since mid September due to an outbreak of the parasitic bug cryptosporidium.
It was revealed that the council did not have a UV filter on its Two Mile Treatment Plant to protect against the protozoa.
There have been 73 confirmed cases of cryptosporidium since the outbreak and 20 probable cases, but just one case in the last two weeks.
Human faecal contamination in the water supply has been identified as the likely cause.
The Two Mile Treatment Plant needs to be cleaned and flushed ahead of a filter being installed.
Queenstown Mayor Glyn Lewers said it would be "quite a large job".
"It's actually never been done before, and we're working with the regulator to come up with a methodology that's satisfactory to both parties," he told Checkpoint on Tuesday.
"We will start flushing one half of the network that the reservoir serves, and that's probably the majority of the residents on the Turn Hill Sunshine Bay side, and then we'll isolate that and then the second half of the distribution network."
Residents will still have water supplied throughout the operation, expected to be completed on 8 December.
"We are still waiting on the regulator to agree to our flushing methodology, so that would be part of the commissioning."
Lewers was not aware of reports some CBD cafes and businesses were not boiling their water.
"I can't speak for individual businesses, but I am, I am assured from the cafes and the restaurants that I've been to the water has been boiled."
The filter, which comes with a $1.4 million price tag, will only be a temporary solution.
"We've ordered a larger unit which is getting made over in, think it's Canada, and that will be delivered and we'll replace the temporary unit and we're hoping to have that in before the end of the financial year."
That will cost $9.4m. Lewers said he was not sure how much the ongoing issue has cost the town in lost business.
"We track visitor spend year on year, and if it is a significant drop, we'll see it."