Fonterra says its latest fortnightly online auction suggests buyers are uncertain about whether milk prices will remain high.
Average milk prices jumped 6% to more than $US4,800 per tonne.
Whole milk powder prices surged 10%, reaching a record high.
Fonterra global dairy trade manager Paul Graves says many buyers have not purchased all their needs at this price, as they may have bought product for the period months in advance.
He says in the later September to November contract, relating to milk that will come in the next season, prices stayed flat.
Mr Graves says that suggests people are not quite so sure about the future and want to wait and see how things develop over the next few months before making purchasing decisions.
BNZ economist Doug Steel says milk prices are likely to fall as supply increases over the year.
He says overseas buyers purchasing New Zealand product at high prices is a major positive for the economy in general, as it usually pushes up the New Zealand dollar.