The Commerce Commission said Fonterra's estimate of risk in calculating the cost of financing milk processing operations is too low, which results in the co-op calculating a higher milk price than its competitors.
The commission said its "emerging view" was that Fonterra's estimate of risk was too low. "The impact of this is that Fonterra calculates a higher milk price than would be the case if it used a more feasible allowance for risk in the cost of finance, consistent with other processors," it said.
The cost of financing - also known as the cost of capital - feeds into the calculation of the milk price Fonterra pays its farmers.
The Commission administers a milk price monitoring regime under the Dairy Industry Restructuring Act (DIRA) as Fonterra has market power over the purchase of farmers' milk.
"For several years now Fonterra has been unable to provide sufficient evidence to convince us that using a lower asset beta than comparable processors is justified," commission deputy chair Sue Begg said.