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