In the first half of 2013 NZ Milk Products' delivered a strong performance on the back of price premiums, product mix, cost savings and productivity gains, he said in a statement.
Fonterra said at the time of its interim result in March that the second half was likely to be more challenging.
"The drought has contributed to a 64 per cent rise in whole milk powder prices on GlobalDairyTrade since early 2013, and this has had a temporary, but significant, negative impact on NZMP's margins," Spierings said.
"At the same time, our Australian business remains under pressure," he said. "Although a recovery plan is being implemented, it is in its early stages and will not counteract the impact on earnings of intense competition and the accelerated reshaping of our business," he said. Changes to the Australian business resulted in a number of additional write-offs, he said.
The revised earnings forecast was subject to continued volatility in dairy prices, foreign exchange and other market uncertainties that might occur in the final month of 2013, he said.
Units in the Fonterra Shareholders Fund last traded on Wednesday at $7.48.