"We believe Fonterra is likely to partially write-down its 18.8 per cent stake in Beingmate, reflecting the downward revision in its earnings forecast to CNY800-1,000 million (NZ$174-217m) and the latest downturn in its share price," OMF said.
"In particular, Beingmate's poor performance has a direct impact on Fonterra's profitability - Fonterra's share of Beingmate's net loss was reported at NZ$42 million in FY2017 - and we expect losses this year to weigh on Fonterra's earnings and dividend payments."
OMF noted recent rainfall in New Zealand had improved pasture growth and suggested the country's milk production may not be hurt by dry conditions as much as Fonterra expected. It expects prices to be flat in tonight's auction after three successive gains.
In late December, Fonterra said it expects its New Zealand milk collection to fall 3 per cent to 1,480 million kgMS this season from 1,525 million kgMS last season as dry weather stunted grass growth.
"In order to achieve this level of output, Fonterra is implicitly assuming that milk collections will be 6.5 per cent lower between January-May compared to the previous season," OMF said.
"We maintain our view that while risks to production remain, we are more sanguine as current weather conditions and outlook appear supportive and we see scope for production in coming months to be more or less in line with the previous season, bringing total collections to 1,520 million kgMS for 2017/18.
"As we believe that the latest rebound in dairy prices can be at least partly attributed to concerns around New Zealand production, should upcoming production statistics surprise on the upside, pricing support could dissipate and we may observe a correction," OMF said.
The brokerage noted that global dairy production remained in expansion, led by Europe.
"We continue to see downward pressure on prices, particularly for cheese and butter, although we are wary that this view has been largely priced in," OMF said.
For the 2018/19 season, OMF said its model suggests a Fonterra milk payout of $6.94/kgMS, ahead of the NZX Milk Futures price of $6.01/kgMS.
"This suggests that there is considerable upside risk to market expectations although we acknowledge that downward pressures remain," OMF said.
Auckland-based Fonterra is the world's largest milk processor and the biggest exporter of dairy products.