Fonterra's Global Dairy Trade-Weighted Index fell 1.4% to an average selling price of $US3580 a metric tonne on Wednesday's auction.
Prices have fallen at the last six auctions, and the latest result is the lowest since November.
However BNZ economist Doug Steel says prices are holding up reasonably well given the turmoil on international markets and currency fluctuations.
Mr Steel says the latest results are unlikely to affect Fonterra's current season payout forecast.
Fonterra last month affirmed its forecast of $7.15 - $7.25, including a milk price of $6.75 a kilogram and a distributable profit of 40 - 50 cents a share.