Pahiatua's residents will have to wait at least another three days before a precautionary notice to boil water can be lifted.
Tararua District Council issued the notice last Friday after a positive reading for E. coli in the lower North Island town's water supply.
The council is doing daily testing and there was good news on Sunday when there was a negative result for the bacteria.
But that turned to frustration on Monday with another positive reading.
There have been no reports of illness in the town.
However, the council says there has to be three days of clear tests before the boil water notice can be lifted.
"We are seeking outside advice and our staff are working in tandem with the appropriate agencies," Mayor Roly Ellis said.
"Ministry of Health has advised that we continue the chlorination of the reticulation and retain the boil water notice."
People have been advised to boil water for drinking, making ice, preparing food, brushing teeth and preparing infant formula.