That meant Grate Cheese and Kaimai qualified as independent processors and were entitled to access the regulated supply.
"All that is necessary to become an independent processor, as defined, is for the party concerned to be intending to process the regulated raw milk it obtains from Fonterra either personally or by means of contractual arrangements entered into for that purpose," the judgment said.
The dispute stemmed from Grate Cheese and Kaimai outsourcing a large chunk of their processing to Open Country Dairy, the nation's second-biggest dairy processor, which had spare capacity to pick up the work.
"To prevent the use of that spare capacity by adopting an unnaturally restrictive interpretation would not be conducive to efficiency in the relevant dairy market," the judgment said.
The judges weren't swayed by Fonterra's argument that allowing its rivals to outsource processing would lead to an explosion of 'virtual processors' tapping the raw milk supply and immediately on-selling it.
"Even if toll processing might arguably in the short run reduce demand for milk at the farm gate, we are not satisfied that this is a factor of sufficient weight, when viewed in a wider context, to overcome both the ordinary and natural meaning of the definition and the general purpose of the act and the regulations," the judgment said.
The decision isn't likely to have an immediate impact on Grate Cheese and Kaimai, with the latter now owning its own processing facility and the former purchasing hard cheese which it then processes and repackages.