He will leave his job at the end of the year.
Brock has been with the bank for more than 17 years, and was part of the team that launched the bank in 2002.
The bank now has more than a million customers.
Chair Susan Macken paid tribute to Brock's leadership through a period of rapid growth and said he had done a great job setting the bank up to continue to expand and prosper.
A search for a new chief executive would begin immediately.
In the last year there have been major changes to the bank including New Zealand Post selling 47 per cent of its shareholding of the bank to the New Zealand Super Fund and ACC.