Pongaroa residents are still having to boil their water, after the Tararua District Council issued a precautionary notice last week.
Peter Wimsett, council's manager of district development, said the problem had occurred following recent heavy rain.
The filter at Pongaroa's water supply at the source in the Puketoi Ranges can be overwhelmed by the volumes of water, diluting the town supply, which is treated with chlorine.
While council can measure the chlorine in Pongaroa, staff can't get up the track to check the intake and discover why the filter is letting too much water through, because of the weather.
"There is a heightened risk because of the number of adverse weather events, with 300mm of rain falling over a 48-hour period in the area a couple of months ago. That was quite extraordinary," Wimsett said.
Council is working towards Christmas to get the plant working, after initially hoping it would open in July.
Wimsett acknowledged people in Pongaroa are wanting council to get the new plant working as soon as possible, but he said it would not open until it was working correctly.
Pongaroa residents who have specific health concerns should consult their doctor or Healthline on 0800 611 116.
People with severely compromised immune systems, infants, pregnant women, and some elderly may be at increased risk. These people should seek advice about drinking water from their healthcare providers.