An economist has done the math on how much water would need to be used to create the $50,000 bill National says farmers will need to cough up under Labour.

Peter Fraser, a former senior analyst at the New Zealand Treasury, told Radio New Zealand that while the average farmer could use enough water to get a $50,000 tax bill, to do this farmers would use as much water as more than 30,000 people.

"Irrigation is the biggest user of water in New Zealand bar absolutely nothing ... statistics put out by the Parliamentary Commissioner for the Environment around five or five years ago showed that 78 percent of the water in New Zealand was actually used just by irrigation," Mr Fraser said.

Mr Fraser supports the tax but wants urban users to be affected as well.

He says he is happy to pay the annual $1.60 bill his usage would generate as long as rural users pay their share as well.

However Irrigation NZ CEO Andrew Curtis disagrees with the statistics.

"Mr Fraser is wrong to say that irrigation uses 78 percent of water. Irrigation actually uses 62 percent of water according to the most recent data from Land and Water Aotearoa. These figures exclude hydro-electric use - if hydro use is included, irrigation use is a much lower proportion of total use," he says.

"Comparing human and farm use of water does not make sense - farms occupy a much larger land area and produce something from the water they use - for example a 100-hectare crop of wheat produces enough bread to feed 10,000 families for a year.

"If we didn't have irrigation, New Zealanders would not have access to such a range of quality local food at an affordable price."

The Labour Party has proposed a tax on irrigators of one to two cents per litre to pay for the cost of cleaning up the nation's rivers.

National has claimed the tax would lead to hugely increased costs for farmers and open up possible new treaty claims at the Waitangi Tribunal.

Newshub.