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Hokitika-based Westland, which was sold to Yili for $588m, recently completed its first year under new ownership.
"As part of this, Brendish has reflected on her personal goals for the next five years and has decided to step down from her role to allow a new leader to take Westland into the future with Yili, the company said in a statement.
Yili also owns Oceania Dairy, in South Canterbury.
Responsibility for Brendish's role will be assumed by Shiqing Jian, resident director for Westland Dairy Co and Oceania Dairy.
Jian said Brendish had been CEO of Westland during its most difficult years.
"Repositioning Westland away from an historical commodities producer to a modern company with hugely successful branded products such as Westgold as well as segregated milk products among other key innovations has ensured that Westland will continue to be part of the New Zealand dairy industry for the next generation,'' Jian said.
"Yili is grateful to Toni for her stewardship of Westland over the past 12 months as we transitioned to Yili ownership," he said.
"It has taken an enormous amount of work to move Westland towards profitability but I am confident that Westland is now in a position to achieve this," she said.
Jian said demand for Westland and Oceania's products was continuing to rise internationally, demonstrating the value that global consumers place on New Zealand dairy goods.
"We are convinced that the future for the dairy industry in New Zealand is a very bright one indeed," Jian said.
Yili said last year that Westland Milk, would form part of its "dairy silk road".
Speaking at a function last August to mark the finalisation of the transaction, Yili Group chief executive Jianqiu Zhang said the acquisition was a significant breakthrough for Yili's global expansion.
"In this regard, the acquisition can be seen as a vast dairy bridge crossing the Pacific Ocean, helping the world to share health,'' Zhang said.
Shareholder farmers in the former co-op received a cash payment of $3.41 per share, a 10-year guaranteed competitive milk payout, plus guarantees that all milk would be collected.