Prem Maan, executive chairman of Southern, said the decision to abstain was the result of careful deliberation.
"Our Westland supplying farms are situated in Canterbury, and as such we are in the fortunate position of having options for who we supply," he said.
"Many of our fellow shareholders in Westland aren't so well placed. So, we believe we have a moral obligation to leave the critical decision on whether the offer should be accepted or not to those Westland shareholders on the West Coast who have and will have no other supply options," he said in a statement.
Maan said Southern felt that Yili had made a fair offer and Southern Pastures would consider working with Yili if they were successful.
"We will, however, be sad to see the demise of the co-operative if that were to happen," he said.
"We are strongly committed to the co-operative model and, in fact, joined Westland and formed our joint venture, New Zealand Grass Fed Milk Products LP, with it because it was a farmer owned co-operative," he said.
Southern is the largest institutional farming fund in New Zealand, owned primarily by European ethical pension fund investors.
The fund says it follows a high ethical code – for example, it has not bought phosphate from Western Sahara for human rights reasons, nor participated in the live export trade of cattle for animal welfare reasons.
Southern owns 50 per cent of specialised dairy company, Lewis Road Creamery.