ANZ's commodity price index fell 2.4 per cent last month, led by a decline in dairy products and logs which make up more than half the trade-weighted basket.
It was its fifth successive monthly drop and left the index 9 per cent below its peak in February.
When converted into New Zealand dollars the decline was 3.5 per cent last month to a level 16 per cent off its high in March 2011 and 12 per cent down on a year ago.
Whole milk powder fell 12 per cent from June and is down 53 per cent on July last year.
"This [was] primarily caused by an overhang of inventory in China and a good production year for New Zealand in 2013/14," said ANZ rural economist Con Williams.