National Australia Bank is conducting what it calls a thorough review of its systems after a third outage in a week left customers unable to use internet and mobile banking.
The lender, whose ATMs, payments and online and mobile banking services had already suffered across two previous incidents, had to apologise again to customers on Tuesday.
Customers were unable to make some payments or access their accounts remotely for several hours.
Systems were restored by 1350 AEDT, but NAB warned customers may still face delays to payments.
"A thorough review of the issues that have arisen in the past week is underway," a NAB spokesperson said.
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"We are committed to improving the performance of our technology and investing in our systems. While the number of issues that impact our customers has declined in recent years, any outage is one too many and every day we focus on finding ways to improve how we serve our customers."
NAB earlier said its ATMs had remained operational and that customers could still use their cards in stores, although some customers airing their complaints online suggested otherwise.
"All services are down can't even withdraw from teller had to leave shopping at the checkout," a user posted on aussieoutages.com.
Nonetheless, Liliane Foederer of DBM Consultants said previous outages at other lenders have shown customers to be generally forgiving of such problems.
"If you've just had lunch and you've got no money to pay for it because your card won't go through, it's a big deal," Ms Foederer said.
"But in general, in two weeks, people will have gotten over it and said 'well, that's part of life".
At 1438 AEDT, NAB shares were flat at $28.28.
