Fish & Game says an urgent audit of illegal rural dumps is needed to establish the scale of the problem of farmers burning or burying waste.
Media reports this week say regional councils suspect thousands of tonnes of contaminated waste are being buried on farms every year, with other waste including plastic being burned.
The councils were reported as saying there was little they could do about the practice.
Fish & Game New Zealand chief executive Martin Taylor said a national audit needed to be carried out by the Ministry for the Environment.
"The scale of this threat is significant. We need to know how much discarded spray, insecticide, oil and paint is being quietly buried in farm pits where it will poison the environment for decades to come.