Labour doesn't have a specified rate for its proposed freshwater tax yet, but has said the highest charge will be "cents per litre" for the best quality spring water.
Farmers would be charged by the cubic metre, or 1000 litres, environment spokesman David Parker said.
And because much of the water they take is lower quality than pristine spring water, the charge would also be lower.
NZ First's cabbage example was "amusing", Parker said, because it suggests that it would take about 1 million litres of irrigation to make "one watery cabbage"
National also took a shot at Labour's water charge.
Primary Industries Minister Nathan Guy said it took 900 litres of water to make a bottle of wine, and a 10c per litre charge would add $75 to a bottle's price.
Parker again pointed out that Labour would charge by the cubic metre.
A tax of 1 cent per cubic metre would therefore lift the cost of a 750ml bottle of sauvignon blanc by less than 1 cent, he said.
National did not back down.
In the absence of any specific rate, Deputy Prime Minister Paula Bennett said, Labour should expect more talk about cabbages until the election.
"Those who have been around a little while know, in politics, that if you do not fill in the detail, others will do it for you."