New Zealand commodity prices fell in January, extending their 12-month slide to about 19 percent, led by casein, wool, beef and aluminium.

The ANZ Commodity Price Index fell 0.9 percent in January, the 11th straight decline. In New Zealand dollar terms, the NZD ANZ Commodity Price Index fell 1.2% in January. The NZD index uses monthly averages for the kiwi and the measure for January didn't fully reflect the currency's sharp decline in the second half.

Casein dropped 14 percent in January, while whole and skim milk powder rose, providing a mixed picture for dairy products ahead of the GlobalDairyTrade auction this week. ANZ Bank Agri economist Con Williams said casein "played catch-up to the rest of the dairy complex, with increased supplies out of New Zealand and Europe weighing on prices."

This week Synlait Milk cut its forecast milk payout to $4.40 per kilogram of milk solids, under-shooting Fonterra Cooperative Group's cut to $4.70/kgMS, citing depressed global dairy prices.

Wool fell 6 percent in January, which Williams said reflected weaker demand in Europe and China combined with a seasonal increase in supply. Beef extended its decline from a record high, falling 3 percent.

Aluminium fell 5 percent, tracking a broader decline in hard commodity prices. Prices were little changed for kiwifruit, apples and forestry, wood pulp prices fell 2 percent and lamb and skins "were up a touch."

Weightings in the index are based on the contribution to merchandise trade. For 2015, dairy accounts for 46.8 percent of the index, followed by lamb on 10.4 percent, logs at 9 percent and beef at 8 percent.