Residents in the Canterbury town of Darfield no longer need to boil their water to avoid becoming sick from E.coli bacteria.
A precautionary warning was issued last Saturday after a sample showed traces of the bacteria which comes from human or animal faeces.
Selwyn District Council began adding chlorine to the town's water supply and continued testing the water.
After three days no further traces have been found.
A council spokesperson says there could be a number of reasons why the bacteria showed up in the single test, including a lab error.