Hokitika-based Westland Milk Products chief executive Rod Quin has given notice of his resignation after seven years in the job, the co-operative said.
Chairman Matt O'Regan said Quin had decided that it was appropriate time for him to seek a new challenge and for the co-operative to appoint a new leader.
He said in a statement Quin had played a critical leading role in developing and implementing Westland's strategy to move into more profitable added-value products - such as infant formula and UHT milk - and so reduce the co-operative's reliance on the highly volatile bulk dairy commodities market.
With its new dryer at Hokitika now operating and the new UHT plant in Rolleston about to be officially opened, the key planks of Westland's value-added strategy were in place, O'Regan said.
"So we support Rod in his decision that now is an appropriate time for him to move on," he said.
The search for a new chief executive would begin immediately and Quin had been asked to remain in the role until his replacement has been found.
"This is genuinely my decision," Quin said. "It's simply time to go and let someone else bring their energy and style to the business," he said.
Westland is New Zealand's second biggest co-operative after Fonterra.