Most of Fonterra's 86 dairy processing plants around New Zealand have been shut down by drought this season - weeks ahead of schedule.

The dry weather has pinched supply to 70% of Fonterra's factories – most north of Taupo – forcing the early shutdown.

Fonterra’s general manager of milk supply Tim Deane says national milk production for the 2009-10 season would still be "a fraction ahead" of last season's collection of about 14 billion litres of milk – thanks to South Island production.

Drought in Northland and Waikato has cut daily milk production back 45 to 55% on last season.

Daily milk production in the lower North Island is 12 to 13% behind last season.

And production in the Nelson and Southland dairying areas was starting to lag behind last year.

But in Canterbury and Southland, milk output is booming.

That’s partly because the South Island dairy herd is up 13% to 2.1 million, according to the 2009 Agricultural Production Survey. That’s 7 times the size it was 20 years ago.

Canterbury was the South Island's largest dairying region, with 10% growth to a herd size of 918,000. Southland cattle grew 19 per cent to reach 589,000.

The size of the North Island herd remains stable at 3.8 million.

All up, national dairy herd numbers reached a record high of 5.9 million last year, up 282,000 since 2008.

Agricultural statistics manager Gary Dunnet says there are good reasons behind the South Island boom. He says sheep numbers have fallen to 32.4 million, deer numbers are down to 1.1 million, and beef numbers remain stable at 4.1 million.