The Commonwealth Bank (CBA) has blamed a "critical security change" for issues with its nationwide payment network yesterday.
The glitch caused otherwise valid MasterCard and Visa transactions to be declined at CBA point-of-sale terminals.
It also caused some machines on the bank's nationwide ATM network to malfunction.
"The Commonwealth Bank made a critical security change to its systems [yesterday] morning," according to a CBA statement to iTnews.
"As a consequence, merchants using CBA terminals have intermittently had valid MasterCard, Visa or other scheme debit card transactions declined at the point of sale.
"There has also been some intermittent issues with our ATM network which has resulted in some machines place limits on cash withdrawals."
Services to CBA's payment networks were expected to be fully restored by 5pm yesterday. The bank apologised "for the inconvenience this essential maintenance may have caused some customers".
A CBA spokesman declined to disclose details of the "security change", citing security concerns.
It coincided with the bank issuing a media release announcing the launch of its NetCode SMS or token service, which generated a secure code to authenticate online fund transfers.
CBA's network malfunction followed issues at the ANZ bank last Tuesday, when a "system outage" left merchants unable to perform EFTPOS and credit card point-of-sale transactions between 11am and 1.30pm.
Other banks impacted?
A CBA customer service representative earlier told iTnews that the bank was aware of an issue affecting "terminals around the country", suggesting that non-CBA ATMs were affected also.
ANZ, Westpac and NAB spokespeople reported no current issues with their payment networks.
Twitter user Regie Gardoce wrote at 2.20pm: "CBA ATM and EFTPOS is down, totally unacceptable level of service."
At 1pm, online store Queen Bee Maternity wrote: "Hi ladies the CBA merchant line is down right now! Sorry, won't be long before it's fixed, but perhaps you can try PayPal instead - thanks!"
One CBA ATM in North Sydney appeared to be working normally for both CBA and non-CBA cards by 2.45pm yesterday.