Leading the total falls in December were milk powder, down $227m, butter, down $62m and milk fats, down $51m on the same month in 2019.
The drop in dairy exports overall was mainly due to a fall in sales to China, it said.
Total dairy exports to China fell $194m (21 per cent) to $740m in December, led mostly by falls in milk powder.
New Zealand-China dairy exports were high running in to the year's end, according to Fonterra's numbers.
The co-op, in its latest global update, said China dairy import volumes increased by 10 per cent or 28,565 tonnes in November compared to the same period last year.
Westpac agri economist Nathan Penny was not reading too much into the December data,
"November was strong, so it's natural that it came back a bit," Penny said.
"When you look at the seasonal pattern and the 12-month total, there is nothing that really jumps out at you. I think it's just a timing issue," he said.
"They are big numbers and if one container ship gets processed a day or two earlier rather than later, then a big chunk of product can fall into one month and not the other," he said.
Penny noted Global Dairy Trade auction prices had started 2021 on a strong note, and he expected prices to tick up again at next week's auction.
ASB chief economist Nick Tuffley said demand from China remained strong, but there were some likely timing issues around export shipments.
"The underlying story still seems to be fairly encouraging," he said.
