An outage to National Australia Bank's EFTPOS and ATM networks is blocking customers from being able to make payments or withdraw cash across the country.
Customers began reporting the problems just before 11am ADST.
The bank notified its users via Twitter of "problems with some of our systems" and apologised for the outage.
Payments by NAB credit and debit cards, as well as withdrawals from its ATMs, are currently unavailable.
NAB advised customers to go to a branch to withdraw cash.
The problems are also impacting NAB's HICAPS payment processing solution for health claims.
"We're working on a fix as a top priority," the bank said on Twitter.
NAB recommended customers request help from the bank via social media to avoid the long wait times resulting from high volumes of calls to its contact centre.
The bank's chief customer officer of banking and wealth Andrew Haggar said the bank was "working hard" to fix the problem, and apologised for the issue.
It is the second time the bank has been left dealing with systems issues in a week.
It is currently unclear whether the issues are related to last week's outage, which took down NAB’s mobile banking, its online UBank subsidiary, contact centre, some payments processing and corporate systems for around 24 hours.
Last week's outage was triggered by a member of NAB's outsourced IBM team deleting a production DNS zone.
Update 1:18pm: NAB said its services were starting to come back online. It did not provide an expected time of full restoration.
More to come