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A spokeswoman for the bank said it was looking into an issue regarding Kiwibank customers who had received a bank statement via email or internet banking with their correct account number, name and address but the wrong transaction history.
"We have written to 4200 customers who received the wrong transaction history via email asking them to delete it immediately and we have deleted all incorrect statements sent via internet banking."
She said the bank was also writing to customers whose transaction history may have been shared with the wrong person, apologising for the error and outlining the steps it had taken to protect their transaction history.
"We believe the impact on these customers is low, but we will work with them to address any concerns they have."
The bank had also contacted the Privacy Commissioner over the breach.
"We have contacted the Privacy Commission to explain what has happened and how we have moved quickly to address this issue."
The state-owned bank is 53 percent owned by NZ Post, 25 percent by the New Zealand Superannuation Fund and 22 percent by ACC.
In February it posted an 18 percent fall in its half-year net profit to $51 million for the six months to December.