Simon Power, the minister of commerce and justice, will take up the mantle of Westpac's head of private banking in January.
Power, who has been a Cabinet minister for three years, is leaving politics next month when the national election will be held.
Westpac says Power will bring a range of skills and experience to the position that dovetail with the focus of its private bank, which serves wealthy personal customers.
“We are enormously excited to be joined by someone with such an outstanding record of leadership and achievement at the highest level," Westpac NZ chief executive George Frazis said.
Power said he looked forward to helping grow the New Zealand private sector.