The Green Party will put a nitrate pollution levy on dairy farmers to pay for the clean-up of the country's rivers.

The party announced its "game-changing" plan for healthy rivers and sustainable dairying at an organic farm in the Wairarapa this morning.

Its levy is expected to raise $136.5 million in the first year, to be spent on sustainable farming projects, preventing pollution in waterways, and reducing risk for farmers wanting to transition to organic and environmentally-sustainable production.

A moratorium will also be placed on new dairy farm conversions, Crown Irrigation investments will be wound up, and the importing of palm kernel will be phased out.

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Leader James Shaw said scientists and environmentalists told the Green Party that New Zealand must reduce the number of cows on farms to clean up the environment.

He said the Greens wanted to stop the causes of water pollution to ensure rivers are clean, rather than just treating the symptoms.

"There's no point spending money cleaning up rivers if you don't look at what's making them dirty in the first place," Shaw said.

"We will invest in the Sustainable Farming Fund, create a Transformational Farming Partnership Fund, support organic farming and growing, and establish a 'Good Food Aotearoa NZ' national sustainability accreditation scheme so that farmers aren't left high and dry.

"Revenue for those ventures will come from a nitrate pollution levy charged on dairy farmers who continue to pollute our soils and water."

Shaw said the Greens would also accelerate depreciation on infrastructure to help farmers pay down debt and free up capital to move away from intensive farming.

"World markets are crying out for sustainably-produced food, fibre and wood, and our dirty rivers undermine the clean, green brand that 80 per cent of our exports rely on.

"Many farmers already know that we can have both healthy rivers and a healthy economy, so we're helping them achieve that."