Chlorine is being added to the Patea water supply this week after the discovery of E. coli in the water.
Residents have been warned to boil water after routine tests detected the E. coli (full name Escherichia coli) bacteria last week.
South Taranaki District Council water supply manager Peter Cook said comprehensive investigations were under way to find the source.
The Patea water supply, with two new bores and a two-celled brand new reservoir, was opened just four years ago in 2012.
Mr Cook said the bores had been built to a very high standard and were very secure.
At this point, as well as adding chlorine and testing the water daily, the reservoir will need to be emptied, he said.
"I had done an external inspection but now we have to drain the reservoir and carry out a thorough internal inspection. You can't take any chances with water. You can't afford to have people become ill."
Mr Cook said because the reservoir was in two cells, each holding 500 cubic metres of water, they could carry out the internal inspection and not leave people without any water, because only one cell would be inspected at a time.
"We will continue to use the chlorine because that kills off any bugs, and we will also be flushing the mains."
In the meantime bringing water to the boil is enough to kill all bugs, however, the council recommends boiling the water for about a minute, he said.
People could notice a difference in the taste and odour of their water over the next couple of days due to the chlorine dosing, he said.
"Chilling the water in your fridge can reduce that taste.
"As a precaution, your water still needs to be boiled, even if the smell or taste of chlorine is present."
Anyone feeling sick should visit their doctor as soon as possible, he said.
The council is in regular contact with the Taranaki District Health Board and is taking advice for precautionary measures.
Further information and updates on the situation can be found on the council's website, www.southtaranaki.com or by calling 0800 111 323.