Power will join the bank in January 2012 replacing Richard Jarrett who has been in the role since 2006.
He announced his retirement from politics in March after driving through a raft of legislation. Power has been in Parliament for 12 years as the MP and in the Cabinet for three years.
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"One of my objectives after leaving Parliament is to contribute to the growth of New Zealand in the private sector and I feel that with my experience in working with and implementing change I can do that," Power said in a statement.
Westpac CEO George Frazis said Power will bring a range of skills and experience to the position.
"We are enormously excited to be joined by someone with such an outstanding record of leadership and achievement at the highest level" Frazis said.
Membership of Westpac Private Bank requires a household income of $250,000 a year and assets worth more than $1 million (excluding a family home).
