A stream has been found running green with dairy effluent from two sources on a Otorohanga dairy farm and one of the discharges appears to be deliberate, says Waikato Regional Council.
The council says it's appalled by the contamination which was found in the stream west of Otorohanga last Friday after a tip-off from a member of the public.
"The council is particularly concerned that one of the discharges to the stream appears to have been ongoing and deliberate," said investigations manager Patrick Lynch.
"We have many farmers doing the right thing by the environment in our region but unfortunately there are still some in the sector who, quite simply, don't seem to care," he said.
Video filmed by the attending officers shows contamination in the stream and the pipe that was put in place allegedly to discharge effluent to it.
Mr Lynch says a member of the public had noted that a stream, that normally runs clear, was running green over several days, particularly early in the morning.
The council's incident response team were at the stream at first light on Friday morning, observed the pollution in the stream and were able to track it back some distance to its source, he said.
"We will launch a formal investigation and, though we remain open-minded about the circumstances surrounding this, it is very likely that enforcement action will follow," Mr Lynch said.