NAB spokesman George Wright told AAP that the Melbourne-based bank's UK subsidiaries, Clydesdale Bank and Yorkshire Bank, had undercharged 18,000 customers STG 19 million ($A32.8 million) worth of interest on their mortgage payments.
"There's no negative or positive earnings impact," he said.
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A computer glitch was made when the Bank of England started cutting interest rates, resulted in NAB's computer systems making errors in the calculations of interest due on mortgages, he said.
Clydesdale and Yorkshire recently finished writing to all affected customers to inform them of pending increases to interest payments so the banks could recoup its revenue.
"They've advised customers that their repayment scheduled will be revised to account for that error which, in most cases, means additional repayments will be required," Mr Wright said.
"The money that would have originally been required to be paid under the loan contract will have to be repaid."
Customers have been given a 60-day notice period, and half of all affected customers will face an interest payment rise of less than STG25 ($A43.14) per month, he said.
Reports in UK newspapers said some customers have secured full compensation.
Mr Wright said he wasn't aware of any compensation claims from customers.
