The Dunedin City Council is testing the water in its Mt Grand reservoir after residents raised concerns about spray drift polluting the water last week.
After residents in the upper Brockville area called the council on Friday, it sent staff out to investigate, council water production manager Gerard McCombie said.
It found a private person was aerial spraying his farmland with Tordon but there was no sign of spraying near the water reservoir, he said.
"It was calm - ideal conditions for spraying."
However, as a precaution the council, had sampled the water and sent it off to be tested for herbicide poisoning.
"We don't expect it to be. We think it'll [water quality] be well above the drinking-water standards."
The council was not drawing water from the reservoir that day.
The Otago Regional Council was also investigating the incident because, under its air-plan rules, spraying beyond the property's boundaries was prohibited.
However, it would be many weeks before the investigation was finished, as the council had to wait to see whether the spraying had killed plants outside the boundary.
