S&P Global Ratings said any wavering of Fonterra's commitment to restore its financial health would put the co-op's credit rating under immediate downward pressure.
The ratings agency said Fonterra's move to write down assets by $860 million and to suspend its dividend was a "painful but necessary" part of the co-operative's turnaround plan.
The impairment charges were "noteworthy" but were non-cash and did not affect Fonterra's fundamental risk profile.
"That said, we forecast the group's underlying earnings for the year ended July 31, 2019, to be materially below the prior year," the agency said.
"Credit metrics could also deteriorate, but to a lesser extent given cash proceeds from asset divestments, reduced capital expenditure, suspension of dividends, and an improved working capital position," it said.