Farmers are turning up the pressure on Labour's water tax proposal as debate heats up over how much it might cost the sector.
The party's plan to charge farmers one or two cents for every 1000 litres they use is being attacked as unfair and likely to take money out of local communities.
Farmers are planning to protest in the dairying town of Morrinsville where Labour leader Jacinda Ardern grew up.
Central Otago farmers have told RNZ they cannot afford a tax on irrigation and it could drive some off their land.
National leader Bill English said Labour's tax could cost farmers up to $50,000 a year - which Labour water spokesperson David Parker described as "scaremongering".
Agricultural economist Peter Fraser said irrigated water accounted for 80 percent of all water consumed in this country and farmers who used it should pay.
Mr Fraser, a former Treasury economist, said if a farmer did pay $50,000 a year in tax, at two cents per 1000 litres, they would be using the same amount of water as 31,250 people.
"Irrigation is the biggest use of water in New Zealand bar absolutely nothing ... so that's an issue we do need to be talking about."
Labour's policy, while not clear cut, had opened up that discussion, Mr Fraser said.
He advocated applying a water tax to everyone, similar to GST.