Labour says $18 cabbages under its proposed water tax are "nonsense" and the levy will raise the $100 million a year needed to clean up waterways.
The party is proposing charging 2 cents for each 1000 litres farmers use - but the details are yet to be hammered out with those involved.
Water spokesman David Parker says claims that would equate to $50,000 a farm are rubbish.
"It would be about $100m across the whole of the country for a year," he told TVNZ's Q+A on Sunday.
"Which is, coincidentally, what (Environment Minister) Nick Smith says we need to spend every year for the next 23 years to clean up our waterways.
"The level of scaremongering around this would make Donald Trump blush. We had Hort New Zealand saying $18 cabbages. That equates to a million litres per cabbage. The thirstiest cabbage on the planet. I mean it's just nonsense."
Farmers would have to pass the cost on to consumers but it would be "a tiny fraction of a cent per cabbage".
"But actually, in respect of milk and cheese, we know that's set by the international price, not locally."
Mr Parker dismissed the impact it would have on farmers' competitiveness with overseas growers.
Someone had to pay for cleaning up polluted rivers, he said.
"Who should pay that? Should we tax pensioners? Or working people? Or should the farmers who are polluting make a contribution?"
Mr Parker said cities had improved their sewage treatment plants - which had cost ratepayers.
"And over those same decades, the rural sector rivers are getting worse. Now, who should pay? Should the polluter pay or should we tax pensioners?"