The Taranaki Regional Council says it is investigating a complaint that a herd of dairy cows has been grazing unrestricted near the banks of the Stony River, near Okato.
The complaint says up to 45 cows were trampling a stream near the river.
The council's compliance manager, Bruce Pope, said the council was making inquiries but to take enforcement action under the Resource Management Act it must find evidence of contaminants entering the waterway.
Mr Pope said staff regularly visited farms as part of their dairy inspection round and where it is likely that stock can enter waterways, farmers are advised to fence them off.
The outcome of the council's investigation will be made public in its incident register.