Dairy product prices slid at the Global Dairy Trade auction, led by a drop in whole milk powder as the outlook for ample supply weighed on demand.

The GDT price index fell 2 per cent from the previous auction three weeks ago.

The average price was US$2,851 a tonne, compared with US$2,885 a tonne three weeks ago.

Some 42,412 tonnes of product was sold, up from 41,945 tonnes three weeks ago.

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Whole milk powder dropped 2.9 per cent to US$2655 a tonne. That's the lowest price since August 2016, according to NZX dairy analyst Amy Castleton.

Regular grade whole milk powder to ship in January plunged a larger-than-expected 6.4 per cent, Castleton said in a note.

"Whole milk powder offer volumes were high at this event, so prices had been expected to come down," Castleton noted.

"There was a greater volume of whole milk powder sold than at the October 16 event though."

"Whole milk powder offer volumes are forecast to peak at the November 20 event," Castleton said.

"The expectations of continued milk supply growth from New Zealand will put ongoing pressure on whole milk powder prices."

At the latest GDT auction, cheddar declined 4.6 per cent to US$3,250 a tonne, while rennet casein fell 2.9 percent to US$5,327 a tonne.

Lactose retreated 2.7 per cent to US$902 a tonne, while butter gave up 1.7 per cent to US$4,045 a tonne, and anhydrous milk fat slipped 1.3 per cent to US$5,044 a tonne.

Bucking the trend, butter milk powder rose 0.8 per cent to US$2,568, while skim milk powder climbed 1.2 per cent to US$1,997 a tonne.

For sweet whey powder, no product was offered or sold, or no price was published for the last event, or on both of the two previous events.

The New Zealand dollar last traded at 66.67 US cents as of 1.15pm in New York, compared with 66.57 US cents at the previous close in Wellington.

There were 142 winning bidders out of 173 participating at the 18-round auction. The number of registered bidders was 517, up from 511 at the previous auction.

"The event dragged out to 3 hours and 2 minutes, indicating there was some strong competition in the market," Castleton noted.