The Waimate District Council found nitrate levels in the Lower Waihao Rural Water Scheme were elevated at 40mg/l on Friday.
The maximum acceptable level is 50mg/l for nitrates.
It was feared the weekend rain would lift nitrate levels further.
Waimate District Council asset group manager Dan Mitchell said the council was actively monitoring the situation so they could advise residents of any changes that could affect the safety of the supplied water.
He recommended those affected sign up to receive email and text alerts through the council’s website.
Water tanks have been available since elevated levels were first recorded in November 2022 and were topped up last Thursday.
Progressing the denitrification plant was complex "given the potential effects on the receiving environment", Mr Mitchell said.
"Council staff have reviewed several options through a technical report and are currently reviewing two further options. Once completed, the preferred methodology for nitrate removal will need to be consented through [Environment Canterbury]."
