An intensive farming practice taking off in the Rangitikei, known locally as spray and pray, is an environmental disaster waiting to happen, according to Fish and Game.
The method involves aerially spraying steep hill country with herbicide, then sowing a winter crop and aerially spraying with fertiliser.
Stock are then fed in mobs on the crop during winter, which strips it back to bare land.
It has locals, including farmers, worried about what it is doing to waterways, particularly the protected Rangitikei River, which is one of 15 rivers in the country with a water conservation order on it.
Fish and Game chief executive Bryce Johnson said he first heard about the practice from a fishing guide in Rangitikei, who was noticing the river was clouding with sediment on fine days and becoming increasingly discoloured.
He said spray and pray was disgusting and pushed the boundaries.
"It's intensifying land that shouldn't be intensified in that way. This is really steep, erodable hill country and it's just asking for an environmental disaster, and that's obviously what's starting to show up."
Labour Party environment spokesperson David Parker said he was alerted to the practice by Rangitikei locals.
He thought it was abhorrent it was being allowed to happen.
"It's just a recipe for disaster and I think it's completely irresponsible of the local regional council not to be controlling that activity."
The practice was not regulated by the Manawatu-Whanganui Regional Council, and therefore not monitored.
The council said the practice came about after its regional plan was devised 10 years ago, but it was in the process of looking at it more closely.
RNZ seriesWater Fools? - running each day this week, on air and online - looks at the troubled state of New Zealand's freshwater. Check here for new stories as they are published.