ASB Bank says more people making online purchases is behind an "unprecedented increase" in dispute inquiries.
A customer of the bank was last week told he would have to wait at least a month to have a fraudulent transaction dealt with because of the increase.
In an email the man was told: "Due to the unprecedented increase in dispute enquiries we apologise for the delay in responding to your enquiry.
"We will endeavour to have your enquiry actioned by 25 May 2018."
The customer found out about the fraudulent transaction on April 24.
A spokeswoman for the bank said it had been experiencing a steady increase in disputed transactions over the past few months.
"The recent increase is due to a number of reasons, driven mainly by an increase in the volume of online transactions, as more people embrace making digital and online purchases."
The spokeswoman said with online purchases it meant fraudsters were ever present, increasing the level of risk, some of which were very complex and took time to investigate.
As well as more fraudulent transactions the bank was seeing more customers disputing charges for transactions like online games, subscriptions they were not aware of, or when a free subscription time had expired and the customer had been caught out.
The increase comes as the banks are facing pressure over how they handle complaints in the wake of damning findings in Australia's Royal Commission.
Last Friday was the deadline for New Zealand banks to present evidence to the Financial Markets Authority and the Reserve Bank to prove they are different to their Australian parents and counterparts.
The ASB spokeswoman said the increase in disputed inquiries was unrelated to the Royal Commission.
The bank said it had increased its resources to bring wait times down for dispute enquiries and expected to have it back to normal in the next few weeks.
"ASB is committed to providing our customers with the best possible service and we have dedicated extra resource to ensure dispute enquiry wait times are decreasing and back to the normal in the coming weeks."
However, the bank is urging people to be mindful of what family members and children are signing up for online using a parent's credit card and to be careful when signing up for subscriptions.
"Make sure you understand the terms and conditions to which you're agreeing before purchasing off a television advertisement or online."
The bank said where fraud was proven it would ensure any interest or charges were reimbursed.
"The cardholder will have no liability if they have not contributed to the loss."