Bad weather has taken a heavy and "highly unusual" toll on Fonterra's milk production as the season heads towards its peak, the co-operative said.

Fonterra, in its regular statement on the amount of product it intends to put on the GlobalDairyTrade auction platform, said its offer volumes had decreased by a total of 12,116 tonnes.

For next week's GDT auction, the co-operative's offer volume had been decreased by 930 tonnes of whole milk powder and 50 tonnes of cheddar.

Furthermore, the Co-operative's forecast offer volumes over the next 12 months for New Zealand products had been cut by 11,136 tonnes, with a 9,086 tonne decrease occurring in the whole milk powder offer volumes over November and December.

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Cheddar and butter offer volumes have also been reduced by 1,000 tonnes and 1,050 tonnes respectively.

"These decreases have been made in response to significant reductions in daily milk volumes across the central and upper North Island in October, particularly the key dairying region of Waikato where early October daily milk volumes have been down around 10 per cent versus the same time in the 2015/16 season," Fonterra said.

"The reduced daily milk volumes are being driven by materially wetter than normal spring weather across most parts of the country, and have resulted in the need to remove wholemilk powder, vheddar, and butter from the production plan," it said.

"These reductions in daily milk volumes are highly unusual and Fonterra will continue to monitor the situation closely," it said.

"Fonterra will continue to assess the on-going impact on future production plans given current expectations that milk collections at the peak of this season will be well below previous season peaks," it said. New Zealand dairy production normally hits its peak around mid October or early November.