The Privacy Commissioner is calling the release of the transactions histories of thousands of Kiwibank customers 'significant' because of the numbers involved and the information that was released.
More than 4000 people had their transaction histories sent to someone else.
Kiwibank is investigating how it sent out the online statements with the correct account number, name and address but someone else's transactions.
The bank says it thinks the impact to customers is low and has written to them apologising for the error.
No one from the bank would be interviewed on Morning Report.
Privacy Commissioner John Edwards told RNZ reporter Charlotte Cook some people will be identifiable by the statements and information sent.