At the latest GDT auction, rennet casein dropped 8.6 per cent to US$5,612 a tonne, while skim milk powder slumped 5.6 per cent to US$1,797 a tonne.
The slide in skim milk powder prices was unexpected, according to Kilsby.
"Skim milk powder prices have been driven by factors surrounding the European-government-run intervention scheme," Kilsby said. "There is speculation that SMP may not be bought in at a fixed price under this scheme next year, which would effectively remove the price support that Europe has for SMP."
Butter declined 2.5 per cent to US$5,736 a tonne, while cheddar fell 0.1 per cent to US$4,107 a tonne.
Bucking the trend, anhydrous milk fat rose 5.2 per cent to US$6,841 a tonne.
"Anhydrous milk fat has been undervalued on a milkfat-content basis compared to butter for a number of months now, so a lift in AMF prices and a fall in butter prices goes some way to correcting this gap," according to Kilsby.
Butter milk powder was not offered at this event.
No prices were available for lactose.
The New Zealand dollar last traded at 71.64 US cents at 1.29pm in New York, compared with 71.71 US cents at 5pm in Wellington the previous day.
There were 137 winning bidders out of 165 participating at the 16-round auction. The number of qualified bidders rose to 544, up from 541 at the previous auction.