"I am flabbergasted by the prosecution, let alone the fine because the council could have said, 'Look, there's a problem here and let's work through this', rather than take me to court.
"In hindsight, yes, there was a weakness.
"The system got blocked but it was fixed straight away and it will never happen again," he said.
In explanation, Purvis, who lives on the farm, said the incident occurred because a sump had blocked with weed in heavy rain and large volumes of water were being used to clean the cowshed, the plant and yard.
The company had earlier admitted a charge laid by the Northland Regional Council of discharging farm dairy effluent into the unnamed tributary and another of contravening an abatement notice at the Hukerenui dairy farm.
Purvis admitted to one charge of discharging dairy effluent.
The judge said the incident occurred because there had been a failure by the systems put in place to deal with the effluent.
"But the reality of it is that as there is a desire to expand the dairy operations and make certain that the income associated with that expansion and the operation grows, there is equally an onus that falls upon the farmer and in this case the company to ensure that the infrastructure that is associated with the farm grows at the same pace," he said.
The judge said he believed an appropriate starting point for a fine was $60,000, less a $15,000 discount for Purvis and the company's early guilty pleas.
After considering the company's financial position, he allowed another $5000 deduction.
Council monitoring programme manager for water and waste, Tess Dacre, said Purvis had been cited twice in 2010 for non-compliance of a similar type that should have served as a warning for him.