Fonterra said writedowns would plunge the co-operative into a $590m to $675m loss in the financial year just ended.
The co-op said it would not pay a dividend for the year to July 31 so that it could pay off debt.
The annual loss would be only Fonterra's second since its inception in 2001.
Chief executive Miles Hurrell said it had become clear that Fonterra needed to reduce the carrying value of several of its assets and take account of other one-off accounting adjustments, which total about $820-860m.
Writedowns are spread across the board.
"Since September 2018 we've been re-evaluating all investments, major assets and partnerships to ensure they still meet the Co-operative's needs," he said.
"We are leaving no stone unturned in the work to turn our performance around.
"We have taken a hard look at our end-to-end business, including selling and reviewing the future of a number of assets that are no longer core to our strategy," he said in a statement.
The review process has also identified a small number of assets that were overvalued, based on the outlook for their expected future returns.
"While the Co-op's FY19 underlying earnings range is within the current guidance of 10-15 cents per share, when you take into consideration these likely write-downs, we expect to make a reported loss of $590-675 million this year, which is a 37 to 42 cent loss per share," he said.
Hurrell said that the majority of the one-off accounting adjustments related to non-cash impairment charges on four specific assets and the divestments that the Co-op has made this year as part of the portfolio review.
"DPA Brazil, the New Zealand consumer business, China Farms and Australian Ingredients' performance have been improving, but slower than expected and not at the level we had based our previous carrying values on," he said.
Fonterra's accounting valuation for DPA Brazil will be impaired by about $200m, due mainly to economic conditions in Brazil.
Closure of its Venezuelan consumer business would mean an accounting adjustment of about $135m relating primarily to the release of the adverse accumulated foreign currency translation reserve.
The carrying value for China Farms will be impaired by approximately $200m due to the slower than expected operating performance.
In its New Zealand consumer business, the compounding effect of operational challenges, along with a slower than planned recovery in Fonterra's market share had resulted in it reassessing its future earnings.
"We are now rebuilding this business and, as part of this, have sold Tip Top which allows the team to focus on its core business. The combined impact is a write-down of about $200m," Hurrell said.
Chairman John Monaghan said that in light of the significant write-downs that reflect important accounting adjustments Fonterra needed to make, the board had brought forward its decision on the full year dividend.
"Ultimately, we are charged with acting in the best long-term interests of the Co-op. The underlying performance of the business is in-line with the latest earnings guidance, but we cannot ignore the reported loss of $590 - $675 million once you look at the overall picture," he said.
"Not paying a dividend for the FY19 financial year is part of our stated intention to reduce the Co-op's debt, which is in everybody's long-term interests," he said.