Westpac's online and mobile banking platforms were unable to process payments or show up-to-date balances for almost a week following an unidentified technical glitch.
The platforms appeared to become buggy and lose transaction histories and current balances on Monday, with customers complaining they couldn't transfer funds or make payments.
Westpac confirmed it was having problems at the time, and attributed them to an "unexpected system outage".
It warned of "an intermittent issue" delaying some funds transfers, and freezing transaction histories and balances.
On Thursday it said the Westpac Live platform was stable and most of the overnight transaction issues had been addressed, but noted that online banking transfers might still be delayed and account balances may not be up to date.
"Fund transfers, BPAY, 'pay anyone' and overseas payments will be processed, but may be delayed," the bank advised.
"Deposits and withdrawals may not always be instantly reflected in your balance.
"We ask you please don't re-submit payments as it may duplicate them."
It promised to reimburse any late or overdrawn fees incurred as a result of the technical trouble.
However, in an update on Friday, the bank said it was still experiencing intermittent issues with available balances and funds transfers.
It said customers could still experience a delay of up to two hours in the processing of payments such as BPay transfers and transfers between accounts.
"Please be assured that these transfers have been processed, but may take some time to be reflected in account balances. For this reason, please don't resubmit payments or transfers," it said.
It continued to advise of delays to the processing of transfers and payments up until Sunday. On Monday it told customers mobile and online banking were running as normal and all payments had been processed, with balances now up-to-date.
The bank has been contacted for detail on the cause of the issue. It has published a customer service page outlining the issues.