Murray Goulburn said its farm-gate milk price of A$5.60 per kg of milksolids was no longer achievable. It now expects a price of A$4.75 to A$5/kg to be paid this year.
In February, the company, which listed its units on the ASX last July, forecast its annual net profit would come in at A$63 million against a prospectus forecast of A$89 million.
In last week's statement to the ASX, Murray Goulburn said it expected its net profit to fall to A$39 million to A$42 million.
The company said it would introduce a milk supply package equating to A$5.47 a kg to protect its long-term milk supply in 2016. Units in Murray Goulburn have fallen by 89 cents or 42 per cent, to A$1.25 in response to the profit downgrade.
The co-operative is the key milk-price setter in Australia.
Fonterra - the second biggest dairy company across the Tasman - warned its Australian suppliers throughout the season about the sustainability of the Australian milk price and advised them to budget conservatively.