Dairy prices fell 5.1 percent in the overnight global auction in the first significant fall since August.
Prices dropped to $US33302 a tonne.
Whole milk powder, a key indicator of farm incomes, dropped 6.7 percent.
Skim milk powder was down 6.3 percent.
Butter fell 2.4 percent but there were rises for cheddar and rennet casein.
Last night's significant fall follows a slight drop two weeks ago, but prices had been rising before then.