A spokesman for Mr Power said that when he stepped down as SOE minister he had no idea what he was going to do but he started receiving all sorts of expressions of interest.
He had settled on the Westpac position only in recent weeks.
Finance Minister Bill English said he couldn't recall when he found out about the appointment, but it was more likely to be in the past month than the past day.
He did not believe Mr Power had been privy to any information that would give him an inside advantage.
Mr Power said he was looking forward to a new challenge and a new environment.
"This is the beginning of a new and exciting chapter in my career. 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."
Westpac chief executive George Frazis said Mr Power brought "a range of skills and experience to the position that fitted well with the purpose and focus of Private Bank.
"We are enormously excited to be joined by someone with such an outstanding record of leadership and achievement at the highest level."
- Herald staff, APNZ