Kiwibank says some of its credit card services have been affected by an issue resulting in transactions being declined.
The issue emerged on Friday after Kiwibank's system failed to register money customers had transferred onto their credit cards.
As a result, some credit card accounts were over limit and transactions were being declined, the bank said.
"We are working urgently to fix this issue and correct all balances as well as reverse any incorrectly charged fees," a Kiwibank spokesperson told Newshub.
The bank said customers only need to make contact should they urgently need extra funds.
"We apologise to impacted customers for the inconvenience and thank them for their patience."
Kiwibank did not say how many customers were affected.