Recent further weakness in general commodity prices such as oil and a reversal in international grain prices following previous gains supported the view that dairy prices will remain weaker than previously anticipated in the near term, it said.
The bank said the wider downward revision to milk price forecasts over recent months had been driven by factors such as ongoing global milk supply expansion, higher-than-anticipated wholemilk inventory in China, US dollar strength, weakness in the euro, the extension of the Russian trade ban on many Western dairy products, general weakness in commodity markets and an associated rise in risk aversion.
Possible reasons for a price improvement from late 2015 could include likely lower production arising from very low prices.
Low prices would also encourage consumption.
BNZ said the drought in California continued to dent milk production from the biggest producing state in the US.
The bank's $3.80 forecast depends on wholemilk powder prices remaining weak in the near term before approaching US$2500 a tonne by mid-next year.
At last week's GlobalDairyTrade auction, prices were at $1848 a tonne, down 13.1 per cent from the previous auction.
Other banks' forecasts for the farmgate milk price vary.
ANZ expects $3.75 to $4 a kg for the current season, Westpac expects $4.30 while ASB is sticking with its forecast of $5.