Labour's proposed water tax is being described as an attack on rural New Zealand.
The party says if it wins the election it will introduce a "resource rental" on water used for irrigation so farmers use it more efficiently.
It's part of a policy to make all New Zealand's lakes and rivers safe to swim in within a generation.
Environment Minister Amy Adams says Labour is proposing a tax on farmers which wouldn't be paid by anyone else.
"It's an out and out attack on rural and provincial New Zealand," Ms Adams said on Thursday.
"Labour's water tax, which they are hiding the amount of, would cause real damage to hundreds of small rural businesses in the productive sector."
Ms Adams says it would increase the retail cost of everything from apricots to cheese.
Irrigation NZ, the national organisation that promotes irrigation, says the policy would be impossible to implement.
Chief executive Andrew Curtis says Labour doesn't understand the complexities of freshwater management.
"If you were cynical you would say this was just about votes," he said.
"It doesn't impact on those in urban areas, so let's put it all on the rural people."