New Zealand's second biggest dairy manufacturer Open Country Dairy is expanding again - this time with a major plant addition in Southland.

Chairman Laurie Margrain said the new plant, currently in the resource consent process, would add 50 per cent to manufacturing capacity at the company's Awaroa site.

It was expected to be operational in September next year.

The planned expansion already had full milk supply, Margrain said.

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He declined to share the cost of the new plant or the milk processing volumes involved.

Open Country, 76 per cent owned by New Zealand's Talleys Food Group and 15 per cent by Singapore's Olam International, recently commissioned a second plant at Horotiu in the Waikato.

The Horotiu site is next door to the headquarters of meat company Affco, owned by Talleys.

Margrain has indicated further expansion in the Waikato is on the cards - either by expansion at Horotiu or at the company's site at Waharoa, near Matamata.

Open Country began exporting in 2004. It exports milk powders throughout the world.