More than 200,000 ANZ customers will share in $13 million in compensation after the banking giant failed to pay them enough interest on their savings accounts.
The corporate regulator says ANZ failed to accurately apply bonus interest to its Progress Saver Accounts for several years.
ANZ reported the mistake to the Australian Securities & Investments Commission after a customer complained in August 2013.
The bank has apologised to its customers, most of whom will receive less than $35 each in compensation.
About 100,000 customers are estimated to be owed less than $5 each.
"Over the past month we have been refunding the amount underpaid to impacted customers, which includes an additional amount of compensation in recognition of the bonus interest not being paid at the correct time," an ANZ spokeswoman said.
"The majority of customers have now been reimbursed, and we expect the refund process will be finalised this week."
It's the second time this year that ANZ announced multi-million dollar compensation packages for customers.
In April, ANZ said it would pay back $30 million to 8,500 customers who paid for priority access to financial advisors, but did not get the annual reviews to which they were also entitled.
In its latest error, ANZ estimates that about 0.5 per cent of 3.5 million Progress Saver accounts were either underpaid or overpaid the correct amount of bonus interest because of a computer mistake.
The bank carried out a review after learning of the errors and implemented changes in April 2014 to ensure the mistakes don't happen again.
"ANZ has taken its breach reporting obligations seriously in this matter," ASIC deputy chairman Peter Kell said.
The mistakes uncovered at ANZ follow similar errors and compensation payouts by its major banking rivals, the Commonwealth Bank and National Australia Bank.
BIG BANK REFUNDS
* Oct 2015 - CBA refunds $7.6m to agribusiness customers for interest rate and fee errors
* Jul 2015 - NAB pays out $25m to 62,000 wealth management customers who were short-changed
* Jun 2015 - Macquarie Investment Management refunds $5.5m to 2,300 customers overcharged in fees
* Apr 2015 - ANZ pays back $30m to 8,500 customers who missed annual reviews from financial advisors
* Aug 2013 - Bank of Queensland refunds $34.5m to 38,000 customers who were overcharged fees and underpaid interest
