The Waikato Regional Council says it has received a wave of complaints about mismanagement of effluent from dairy farms.
Two have led to prosecutions, while six more are still being investigated.
The council said in one case, P Walter Farm Limited, near Pukekohe, was fined $53,440 at Auckland District Court.
The council said staff found effluent irrigators had been deliberately disconnected, which allowed an effluent pond to build up, and in one case, discharge into a paddock for five days.