The Green Party has drawn on Ministry of Agriculture farm profitability statistics to defend its proposal for a charge on irrigation users to help pay for cleaning up polluted waterways.
It says its suggested charge of 10 cents per 1000 litres of water would raise up to $570 million a year.
Irrigation New Zealand says it would impose an unfair burden on commercial water users.
It has calculated that it could cost a 210-hectare dairy farm in Canterbury up to $105,000 a year.
Green Party co-leader Russel Norman says recent farm profitability statistics released by MAF show the charge would be less than 5% of total income for the average Canterbury dairy farming operation.
He says the statistics show that after paying the suggested charge for irrigation water, Canterbury dairy farmers would still receive on average more than $500,000 in farm profit before tax.
Horticulture New Zealand has also criticised the irrigation tax idea, saying it would be a quick way of putting a large number of fruit and vegetable growers out of business.