Federated Farmers' challenge to Labour is: "Tell us what numbers you have in mind."
Labour last week announced proposals for a tax on water for large commercial users, including farmers who rely on irrigation water, but in the absence of detail some eye-watering numbers in the billions of dollars have been floated.
Federated Farmers water spokesman Chris Allen said the pledge to consult with those affected if Labour is part of the new government is appreciated, but it still means voters are sailing blind into the election.
"Surely at least they have a starting figure in mind, to open negotiations with water users and to give voters a clue on the quantum of tax they envisage.
"Ten cents a litre has been suggested for exporters of bottled water, which has been contested as a thousand times exaggerated for the royalty that might apply to large commercial users.