The Reserve Bank estimates that 80% of an official cash rate hike will have been passed on to mortgage holders within about six months.
Harmoney will take up to two years to refund 37,000 customers.
The number of Bonus Bonds on issue fell to 778,318 at March 31 from 3,125,672 a year earlier.
Kiwibank's profit growth was strong even before the write-back of charges for bad debts, rising 20% before tax.