ASB said volumes will reach their seasonal lows in the next auction.
ANZ said its forecast of a milk price in the low to mid $7/kg range remained unchanged.
Westpac said auctions could be volatile from fortnight to fortnight but that the latest result introduced downside risk to its $7.10 forecast, which was predicated on WMP prices averaging around US$4000 a tonne.
Westpac senior currency strategist Imre Speizer said that before the latest auction, markets had factored in lower dairy product prices.
He said "last night's fall came as a genuine surprise".
Weaker dairy prices played a big part in the ANZ Commodity Price Index posting a third monthly decrease in May.
The index slipped 2.2 per cent in the month, to be 5.8 per cent below the record high posted in February, with dairy leading the falls.
ANZ said it wasn't all one-way traffic. Seven commodity prices recorded a lift in May, led by a 5 per cent increase in wool prices.
See the latest ANZ commodity price index here: