The short-term price, for shipments in September fell 8 per cent to $US1841/tonne, reported to be a five-year low.
Further out, for shipment in October-December, prices at auction fell 4.6 per cent to $US1806/tonne.
But there was a brighter note, with the price on shipments between January and March next year lifting 1.9 per cent to $US1849/tonne.
At last month's auction, the longer-range price fell 14.2 per cent to $1814/tonne for December-February shipping.
In June, the average price at auction fell 12 per cent, overall, after slipping more than 4 per cent in May.
Fonterra has previously suggested that prices are bouncing around their bottom range before recovering next year.
The past three auctions have destroyed the only gains so far seen in the online sales, a 3.5 per cent lift in April, following a 16.6 per cent rise in early March.
The latest fall has revived speculation Fonterra's forecast payout for the current milking season could be cut from $4.55/kg milksolids.
This payout figure is down 12 per cent on last season, with Fonterra blaming weak demand.
It has also criticised renewed export subsidies being paid by the United States and European Union for dairy produce.
Blue Read, chairman of Fonterra's shareholders' council, told Radio New Zealand there were "a fair number" of the cooperative's 10,500 farmers exposed to losses at the $4.55 payout level.
If the forecast payout fell further, "I don't think there's much insulation left".
Source: AAP NewsWire