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The appointment completes the requirement of the company's constitution for three independent directors on the board upon listing on the stock exchange, which is planned later this month.
Mr Knowles joins former Fletcher Building Ltd chief financial officer Bill Roest, and former dairy industry executive Graeme Milne.
Mr Milne, who is chairman of the Synlait board, said Mr Knowles' experience in establishing and ''growing'' Kiwibank would be valuable to the company as it implemented growth initiatives expected to cost about $183 million.
Since leaving Kiwibank after 10 years as founding chief executive, Mr Knowles has become chairman of listed online accountancy services provider Xero.
He is also a director of SLI Systems and TrustPower as well as chairman or director of various unlisted technology companies.
He is also a director of Rangatira, a Wellington investment company trading on the unlisted platform.