Green Party leader James Shaw unveils a policy to tax pollution, and reinvest money into sustainable farming practises in a bid to clean New Zealand's waterways.

The Greens are promising a tax on pollution, set to raise $135 million to be reinvested back into sustainable farming.

Green Party leader James Shaw said the policy would introduce a nitrate pollution levy charged on dairy farmers who "continued to pollute our soils and waters".

"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 on Saturday.

MONIQUE FORD/STUFF Green Party leader James Shaw says the Greens would fund a package of "game-changing support measures" farmers could use to reduce their impact on the environment.

Shaw said the revenue from the levy on nitrate pollution from agriculture would raise about $136.5m a year, starting with intensive dairying, and would fund a package of "game-changing support measures" farmers could use to reduce their impact on the environment.

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The levy would initially be set at $2 per kilogram of nitrate that was lost to land and water per hectare of farm, per year. Initially, the levy would apply only to dairy farms but a "fair pollution levy" would be extended to all forms of agriculture and horticulture over time.

"Dairy intensification over the last three decades is directly linked to rapidly declining water quality," Shaw said.

In other measures, the Greens would extend the Sustainable Farming Fund with an extra $20m a year and invest $210m over three years to create a Transformational Farming Partnership Fund, to focus on issues such as farming for clean water and adapting to climate change.

The party also promised to increase funding to the Landcare Trust to $16m over three years, reward tree planting by farmers and landowners, allow accelerated depreciation on dairy farm equipment to help farmers free up capital, and support organic farming through a new national certification scheme with new funding of $5m a year.

The party's environment spokeswoman Eugenie Sage the tools to monitor nitrate levels across farms were already being used by councils. It wasn't without flaws, but the technology was in place to ensure pollution would not go unchecked, but also be attributed to the responsible parties.

It wasn't about "pinning farmers to the wall", Shaw said.

Many were responsible and investing heavily in sustainable practises already. They would not fax any additional tax. And those who might face a loss of income due to the tax would be supported by the fund to shift to more sustainable practises.

The party also wanted to see farmers shift away from increasing herd sizes to types of farming that led to more value-added products.

Responding to the Greens' policy, Labour leader Jacinda Ardern said Labour would not try to cut the number of dairy cows but would look to manage the growth of the dairy herd.

In some areas councils were putting in place a process for those who wanted to do large scale intensifications, and she favoured a more national approach.

"If we continue to expand it is going to have an effect on our environment."

Speaking to reporters after a street corner meeting at Kingsland in her Mt Albert electorate, Ardern said of the nitrate charge: "obviously that's their policy, it's not included in our policy".

it would be a matter for "conversations that we need to have after the election" if they were in Government with the Greens.

She also backed the Government's Predator Free by 2050 policy as a 'laudable" policy, but she said it needed to be resourced properly.

On Saturday Prime Minister Bill English announced another $69m in conservation funding to go towards achieving the target.

Federated Farmers have branded the Greens' policy as "discriminatory".

"If this is the kind of 'help' we can expect farmers to get from the Greens, then New Zealanders simply can't afford to continue to rely on the primary sector for our economic viability," said water spokesman Chris Allen.

"The Greens' approach is out of touch with, and an insult to, the collaborative work been undertaken by farmers, particularly over the last decade, and indicates that this party simply doesn't get it."