The Environment Court has told a regional authority it cannot abandon its responsibility to maintain water quality.
Ngati Kahungunu took legal action against Hawke's Bay Regional Council, fearing officials would allow contaminants to run into the Heretaunga and Ruataniwha aquifers.
The iwi contended that the council planned to allow groundwater quality to be degraded by giving approval for agricultural run-off to leach into the groundwater.
The tribe took the council to court and won.
The Environment Court said it was a function of every regional council to control the use of land to maintain and enhance the quality of the water.
It said that function was not optional; it is something a regional council is required to do.
Hawke's Bay Regional Council said it would respond to the court's finding tomorrow.