Clyne said leading more than 42,000 NAB employees was the most rewarding role of his career but that the job had taken a personal toll.
"I am leaving to spend some much-needed time with my young family."
Like Thorburn, Clyne took over as chief executive during the global financial crisis and had to develop an effective strategy to steer the bank through a challenging period, Chaney said. Thorburn agreed the BNZ had become something of a springboard for NAB.
"The chief executive of the BNZ is the CEO of a full bank and in banking these days that's quite unique."
Thorburn added the New Zealand banking scene was highly competitive, which required banks to consistently strive to maintain a sharp edge.
His base salary at NAB will be A$2.2 million ($2.37 million), theoretically rising to A$9.21 million if performance targets are met.
BNZ chairman John Waller said Thorburn had shown inspirational leadership of the bank at a truly challenging time, beginning in September 2008 as the global financial crisis was unfolding.
Melbourne-born Thorburn said the crisis was "a perfect storm of ugliness" when it looked doubtful that the bank would be able to roll over existing funding facilities. He said the Christchurch earthquakes, which affected the bank's 400 or so staff and thousands of customers, were also extremely challenging.
Under Thorburn's management, BNZ's cash earnings increased from $557 million in 2008 to $788 million last year. Deposits increased from $24 billion to $38 billion.
Before heading the BNZ, Thorburn was executive general manager retail banking at NAB for three years. He is a career banker who has spent more than 27 years working in the industry in Australia and New Zealand.
In NAB - Australia's fourth-biggest bank - Thorburn will inherit a strong New Zealand and Australian operation but will still face a problematic British banking, arm - Clydesdale Bank - which has consistently failed to perform.