New Zealand First says a North Otago farming couple's dung dumping protest against the Otago Regional Council last week highlights the inadequacy of the Overseer nutrient management system that councils are using as a regulatory tool.
The Regional Council has rejected this claim.
Overseer is a computer software package originally designed to help farmers assess nitrogen and phosphorus losses and greenhouse gas emissions from their farms.
Regional councils are now using it to set nutrient discharge levels in their land and water plans, despite its much publicised limitations.
New Zealand First's Primary Industries Spokesperson Richard Prosser said those limitations are a factor in North Otago farmers Robert and Sylvia Borst's ongoing dispute with the council over new nutrient discharge restrictions, which they say will force them to stop farming.
Last week, out of frustration, the couple dumped a load of horse manure in the Regional Council's driveway.
Mr Prosser said the Council has based its new rules on the Overseer system that is not up to the task, and is throwing up error rates of between 30 and 100 percent.
"The single biggest issue is the fact that Overseer isn't calibrated for all soil types in New Zealand and for all types of land use - it needs to have some additional investment put into it, in terms of paying for expert people to make sure that it is properly calibrated, so that it can be used as a device that's able to give accurate readings as to how much nutrient comes out, relative to what goes in.
"No one should be using this thing as a baseline for setting rules and regulations at the moment, because it's simply not up to speed for that.
"It can be made up to speed with what in relative terms is a very, very small amount of money."
He believed it would take only five or six million dollars of Government funding to bring Overseer up to speed.
But the Otago Regional Council's chief executive Peter Bodeker said Overseer was not to blame for the Borsts' predicament.
It was because they were in a sensitive acquifer area requiring more restrictive nutrient discharge limits.
"The Otago water plan says that farms need to have up to a maximum level of nitrogen flowing through the soil profile," he said.
"In Mr Borst's case, he's looking to find a way to reduce that, to become compliant, and it's about being compliant with the rule by 2020."
"For most farmers in our region, the maximum level of nitrogen that can be put through the soil profile is 30 kilograms per hectare, per year, based on Overseer."
Mr Bodeker said he didn't think the Overseer model was as wildly inaccurate as some were claiming and he said it was being improved.
The latest upgrade is said to take better account of irrigation in estimating nutrient levels.