ASB apologised to customers for the errors.
"This error occurred when customers received an indicative calculation of what the early repayment amount would be if they were to repay their loan early, but this amount was not recalculated and updated at the time the loan was repaid," the bank said in a statement.
"We have made changes to our processes to ensure calculations are up to date going forward.
"Most customers who were overcharged have now been contacted and received a payment reimbursing them for the amount overpaid, plus an additional amount recognising that this money should have been available to them."
ASB has also provided the Commerce Commission with a plan explaining how it will locate affected customers to make the refunds.
Affected customers will be paid the difference between the amount that they should have been charged and the amount that ASB actually charged them, together with interest.
ASB began repaying customers in July 2020 and has already repaid a significant proportion of the $8.9m to affected borrowers.
ASB identified the issues and reported them to the commission.
The calculation errors affect more than 48,000 early readjustment fees for borrowers who terminated their fixed-rate term loans between April 2005 and December 2016.
"Lenders must implement robust systems and processes to help them to comply with their legal obligations. Those processes should include adequate checks to identify and rectify failures quickly if they arise," said Commerce Commission chair Anna Rawlings.
"ASB identified these matters itself and reported them to the commission. It developed a plan for compensating borrowers and it has begun making refund payments to affected customers. It has waived any claim in relation to undercharged ERA fees. ASB has also made process changes to ensure the problem does not reoccur."
