The average price at the overnight auction fell by almost 3 percent to $US3176 a tonne.
Following a 4.7 percent fall at the previous auction two weeks ago, it was the lowest price since January last year.
The price of wholemilk powder, which strongly influences the payouts for local farmers, fell by 2.6 percent to $US2966 a tonne, a six-month low.
China is a large buyer of New Zealand dairy products.