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.
In another case, the Hamilton District Court found mismanagement of effluent on a dairy farm operated by Allan Crouch at Orini, resulting in a $51,750 fine.
The Waikato Regional Council's investigations manager Patrick Lynch said it was investigating six further complaints made in the last few weeks.
These related to alleged discharges into streams near Hamilton, Karāpiro, Te Awamutu, Otorohanga, Gordonton and Te Pahu, Mr Lynch said.
"Each of these incidents is serious and is being investigated," he said.
Mr Lynch said while the council appreciated this was a busy time of year for farmers, effluent management must be a priority.
He said the wider farming industry had worked very hard to reduce their environmental impact through riparian fencing and planting, but unfortunately there were still some farmers who continued to let everyone down and continued to pollute.
"We are very grateful to the community members who have contacted us. They have seen something that does not look right and called us, enabling us to respond and halt the pollution as quickly as possible," Mr Lynch said.