It is the first price change from the processor since February.
Muller has announced its standard milk price will reduce by 0.5ppl from June 9 to 26.19ppl before the additional retailer supplement.
Milk prices have been held in recent months and the move was the first price change since a 1.25ppl increase in February.
Agriculture Director, Rob Hutchison said: “Whilst the nature of our business and focus on adding value protects our farmers through a more stable milk price than one driven by the commodity sector, we must ensure that we reflect key changes in the market environment.
“We have been monitoring supply over the lead up to the peak period and our direct supply volume continues to be very strong and significantly ahead of the same period last year.
“We have worked to minimise the scale of this price reduction whilst responding to the balance of supply and demand driving the current market value for milk.”
“We have reviewed our position with the MMG Board who are understandably disappointed by a price reduction but understand the market environment we are working in and our commitment to pay a competitive milk price and be clear with our communication to our farmers.
“We will continue to work with the Board to review our position.”
Muller has also confirmed almost all of its 1900 farmers have signed up to its new contract.
A spokesman for Muller said: “We can confirm the contracts process is all but complete. We are very pleased with the discussions with farmers.”