The Grey District mayor says the cause of contamination to one of its water networks may never be found, describing the situation as "looking for a needle in a haystack".
The West Coast town of Greymouth and surrounding areas have been under a boil water notice since last Wednesday due to contamination in one of the district's five water supply schemes.
The council had been on course to lift the notice by the weekend with consecutive days of clear test results on Friday and Saturday.
However, low levels of coliform bacteria, often linked to human and animal waste, were found at Taylorville and Rapahoe on Sunday.
Council staff have been investigating the potential source of the contaminants over the past week.
Mayor Tania Gibson said the situation remains unexplained.
"It's a bit like looking for a needle in a haystack at the moment," she said.
"We're hoping we may find out and we have our resources thrown at it.
"At the end of the day we might not get an answer. It is very difficult to track down what this is."
The low levels of coliform that were found, in isolation, wouldn't typically trigger a boil water notice, Gibson said.
But under the Water Services Act 2021, the council needs to keep the notice in place until three consecutive days of clear testing have been completed.
Gibson said the council's initial enquiries explored contaminants from animal waste as a potential cause, but it was still awaiting the results of this.
The latest round of sampling had clouded the situation even further, she said.
"We all have deferrals in our infrastructure that could be upgraded," she said.
"But normally that's easier to track down if there's a problem in the infrastructure."
Taumata Arowai's head of operations Steve Taylor said it had been in regular contact with the council and would continue to monitor the situation closely.
"The supplier must ... investigate across the supply to find the cause of the contamination and be satisfied that the remedial actions taken have been effective to prevent recurrence and that the drinking water no longer presents a risk to health."
Although the community had adapted over the past week, there was a sense the boil water notice was starting to take its toll for some.
Paroa Hotel owner Alan Monk has been driving back and forth between Greymouth and Hokitika to get safe drinking water.
He said he hoped the problem would be quickly rectified.
"It seems the readings are getting lower so it seems to me that things are improving," Monk said.
"Hopefully they are ... because it's getting pretty frustrating having to provide water bottles and continually propping up our water supply for our coffee machines.
"It's starting to get a bit tedious."
Greymouth's Te Nīkau Hospital and Health Centre was faring well through the challenging period.
Health New Zealand's associate director of operations for Te Tai o Poutini West Coast Philip Wheble said the hospital was coping with the challenges presented by the boil water notice.
"Bottled drinking water continues to be supplied to all inpatient areas and will be topped up as required," he said.
"With sufficient bottled water supplies coming from Canterbury each day, as well as disposable items there is currently no concern for provisions."
