National Australia Bank (NAB) announced it was refunding around $25 million to around 62,000 customers after identifying errors and processes on its Navigator platform dating back to 2001.
Following a request from the Australian Securities and Investments Commission (ASIC), NAB commissioned PwC to conduct an independent review of Navigator, which found errors related to how income and tax was being allocated to customers' accounts upon closure.
As a result, excess money was held within the platform, benefitting fund customers instead of at the individual customer account level. But the money was never held as part of NAB group's assets, the bank said.
NAB Wealth group executive and chief executive of MLC, Andrew Hagger, said the bank would write to customers and advisers over the coming weeks to explain the issue and the solution NAB would implement.
"These errors are in no way related to the quality of NAB Wealth's advice to its customers," Hagger said.
"These errors date back to 2001 and are centred on processes and controls relating to Navigator - a platform NAB inherited when we acquired Aviva in 2009.
ASIC Commissioner Greg Tanzer said: "ASIC expects banks to vigilantly monitor their platforms for issues such as this."
"Any issues identified should be swiftly and pro-actively reported to ASIC, with a view to promptly compensating customers.'
The average payment per customer is $400, including interest.
One third (34 per cent) of customers will receive $50 or less, 50 per cent will receive less than $100, and 75 per cent will receive less than $350.
Customers receiving money are those who closed their accounts on the platform between 30 September 2001, and 30 April 2015.
Money distributed to customers will be done so within the platform funds, which makes the payment immaterial to NAB, the bank said.