Customers withdraw money from National Australia Bank (NAB) Automatic Teller Machines (ATMs) in central Sydney, Australia, July 24, 2015. Reuters/David Gray

Another service outage hit the National Australia Bank (NAB) on Tuesday morning, the second within a week. However, the system has started to come back online and bank customers could now withdraw cash from some ATMs and make EFTPOS transactions.

But not all NAB customers could use their NAB cards in ATMs or EFTPOS terminals because the outage also affected some NAB merchant terminals and HICAPS, the bank said in a statement. The inconvenience caused some NAB customers to rant about the outage on social media, The New Daily reports.

NAB assured customers that its branches are open and they could get assistance there. The lender says it is working hard to fix the problem and it apologised to customers for the trouble the outage is causing them.

The bank promised to continue to provide customers with regular updates. The cause of the outage remains unknown. Until the problem is fixed, Andrew Hagger, chief customer service officer of NAB, urged clients to continue using branches, Adelaide Now reports.

While branches are operational, the wait could be as long as 20 minutes, disgruntled customers posted on social media sites, such as Twitter and Facebook. Jae Gardner recalls the embarrassing experience of the transaction being cancelled and having no money to pay for parking.

Another Facebook user from Yarra Valley writes that high winds hit power lines, causing power outage. But Jan Amos points out they could not use their NAB cards to pay for things they need.

The first outage, which hit NAB on Wednesday, was caused by an IBM software error. These frequent outages create a negative impact on consumer trust, points out Paul Mazzola, a former bank executive and finance expert at the University of Wollongong.

“They need to trust the banks to ensure the service and network are always operational because banking is a daily need and even if the system is down for one day, that interrupts the flow of transactions we all depend upon,” Mazzola warns.

He adds if the outages occur on a frequent basis, customers may consider leaving their lenders.

An NAB spokesperson issued the following statement regarding the outage:

"NAB services are back online following an outage earlier today.

"There may be a delay in the receiving of some payments and we’re working to process these as soon as possible.

“Thank you to our customers for your patience, we understand that this has been inconvenient.

"Our teams are working hard to monitor our services and ensure the stability of systems for our customers.

“A thorough review of the issues that have arisen in the past week is underway.

“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.

“Any customers needing help can contact us at”