Fonterra's deviation from its usual pricing practices is being looked at by the Commerce Commission.
It's just begun its second annual review of the way Fonterra calculates prices under the Milk Price Manual.
Commerce Commission Deputy chair Sue Begg says Fonterra's forecast payout this season is below what it calculated.
She says they haven't seen this situation before, so will focus on it.
Fonterra has advised the Commerce Commission it's because demand for whole milk powder outstrips their production capacity.