New Zealand commodity prices fell at their fastest pace on record in July, notching up a fourth straight month of declines to the lowest level in almost six years, led by dairy.

The ANZ Commodity Price Index for July fell 11.2 per cent to the lowest level since October 2009, and is 27 per cent below year earlier levels. Thirteen of the 17 main commodities declined.

The drop was led by a slump in dairy prices, which are down 23 per cent from June to the lowest level in more than 11 years. Whole milk powder prices fell 35 per cent, while skim milk powder declined 11 per cent and casein dropped 9.8 per cent. Cheese and butter prices also declined. Excess global supply and lacklustre demand is pushing down prices for dairy products, the country's largest export commodity, reducing farm incomes and weighing on economic growth.

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"International supply and demand imbalances remain as a significant headwind for dairy exporters," said ANZ Bank New Zealand agri economist Con Williams.

Elsewhere, aluminium prices fell 2.6 per cent in July, and forestry prices slipped 2.2 per cent amid softer Asian demand, ANZ said. Horticulture prices fell 1.1 per cent as kiwifruit and apple prices declined.

Overall prices for the meat and seafood sectors were unchanged.

Beef prices recorded the only increase across the main commodity groups monitored by ANZ, helped by a strong US market. Beef's 5.4 per cent increase offset a 2.7 per cent decline in lamb, a 4.2 per cent fall in wool and a 2.9 per cent decrease in skins.

The NZD index fell 6.7 per cent in July, the third-largest monthly fall since the series started in 1986. The trade-weighted index declined 3.6 per cent over the month, helping mitigate a "gargantuan fall" in world commodity prices, but the local currency is not moving fast enough in response to shifts across commodities, ANZ said.