Close to $10 million will be credited to around 100,000 ANZ customers following a loan payment mix-up.
Home, personal and commercial loan customers will be credited a share of the sum after an error was found in the ANZ loan calculator.
Following changes to the consumer lending legislation in 2015, ANZ introduced the loan calculator to calculate customer repayments and loan terms when customers asked for changes, ANZ spokesman Stefan Herrick says.
"Unfortunately, we found out in June 2016 there was an error in the calculator's code which meant we left out some interest customers were due to be charged after their loan was changed," he explained.
"We implemented a correct, independently tested calculator in May 2016.
"We identified the problem, fixed it, informed the regulator and are putting customers right. No customers will be out of pocket."
Deloitte helped ANZ review the issue, developing a plan on how best to help impacted customers.
"Once we had a clear picture of what had happened, how our customers were affected and how we would put it right, we contacted the Commerce Commission in June 2017. We have been openly engaging with them as we've worked through putting our customers right," Mr Herrick says.
The issue arose as the calculator didn't account for the accrued interest between the first day of the month and the date the loan was changed during that month.
In most cases, customers were paying slightly less interest than they should have been between the date the loan was changed and when they next changed their loan.
The error resulted in no more than a month's interest, Mr Herrick says.
Each customer's repayments would have increased on average by $2 a month.
"We don't want any customers to be disadvantaged by this, so, we're crediting some interest back to customer's loans. This means customers will pay less than what they had agreed to under their loan agreement with us," Mr Herrick explained.
"We're putting customers back into the position they would have been if the error in the calculator hadn't happened, with ANZ wearing the cost while their loan was impacted."