Commodity prices fell 2.8 percent in the 12 months to the end of April, the first annual decline since February last year.
ANZ's Commodity Price Index is now back to the level it was at eight months ago.
Month on month, prices fell 4 percent in April, largely thanks to falling dairy product prices.
Butter was down 14 percent, skim milk powder 12 percent, whole milk powder 10 percent and cheese 4 percent.
Offsetting that, aluminium prices rose 6 percent and sheepmeat prices 3 percent.