The Commonwealth Bank has apologised to its customers after a major glitch left them unable to make purchases with their bank cards or access their accounts for hours.

Customers received error messages when trying to use the NetBank online banking service and the CommBank mobile app.

Some told the ABC it meant they could not pay for essentials like fuel, medications or rent.

Some users also reported problems accessing the bank's financial services arm, CommSec, and other said direct debits for bill payments failed.

Bundaberg woman Jade Bury's card kept declining at a doctor's appointment this morning.

When she contacted the bank's customer service team via Facebook, she was advised to try borrowing money from family or friends.

"I'm just absolutely livid. I don't think it's good enough," she said.

Ms Bury could not pay for her appointment, and she said she would have to go without medication she needs for a bad infection caused by tonsillitis.

"I had to walk away and say I can't pay for my appointment because I didn't have the funds," she said.

"I couldn't access anything … it's quite frustrating."

What caused the CommBank outage, and when will it be fixed?

A spokesperson told the ABC said the outage was caused by an update to an "internal application" used by the bank, which then caused a problem with a server.

In an update posted at 4:00pm AEST, the bank apologised to customers and said:

"Services are recovering following intermittent issues that some customers experienced across our banking services today.

"We can confirm we have identified the cause of the issue and services are being restored.

"Please note there may be delays in some payments as we continue to bring services back online."

Some customers told the ABC they started regaining access to their online accounts in the afternoon, and others were able to make payWave purchases after lunchtime.

However, some users still reported issues across the afternoon.

According to Downdetector, which tracks online outages, hundreds of people were still logging problems with the website and mobile app as of 3:00pm.

Commonwealth Bank did not answer the ABC's questions about how many customers were affected.

The bank is Australia's largest, with more than 15 million customers across a range of services.

About 7.7 million people use the CommBank app, the bank said on its website last month.

Bank branches were open on Monday, but the bank warned customers that they would be busy.

A sign tacked to the window of a Sydney branch said cash withdrawals were being limited to $300 per customer.

Commonwealth bank customers 'livid' after payments decline

Many customers told the ABC they did not know about the banking outage until they tried to pay for something.

Christopher Curr, who runs a lawn and garden business in Cairns, found out about the outage when he tried to pay for fuel this morning.

"Wasn't able to pay on the card, just kept declining, even though I know there's funds in there so I ended up just paying with cash just to obviously get me through" he told the ABC.

"If I didn't have cash on me, I would've been stuck at the fuel station ... that would have been a nightmare."

He said he did not know if his clients could pay him, and he could not buy material he needed for work.

"I've tried to order mulch for a job on Wednesday — same kind of thing, card kept declining," he said.

"So I'm hopeful it's going to get fixed either today or tomorrow so I can order those materials to do my job."

Ms Bury believes the bank should have notified customers directly via text message, email or the banking app, so that "people like me aren't going to get a shock when we're trying to pay for something".

She plans on changing banks as soon as she can and believes many could join her.

Other customers have also taken to social media to air their frustrations, with several sharing Mr Curr's experience of being unable to pay for fuel.

"I am stuck at a petrol station and cannot pay for fuel!!!! My app is not working - how should I pay??? I cannot use my own money, what a disgrace CBA," one Twitter user said.

Earlier in the day, customers calling the bank's telephone banking service were greeted with this pre-recorded message:

"We currently have a technical problem with telephone banking and NetBank, which will prevent you from transferring money and paying bills. We're working to resolve this issue as a top priority and apologise for any inconvenience. You may wish to call back later as we are experiencing a high number of calls."

Gerard Brody, the chairperson of the Consumers' Federation of Australia, said the technical outage highlighted some of the problems with moving to a cashless society.

"People are reliant on banks to make sure they have their systems right so it's incumbent on banks to compensate people not only for any financial loss, but any severe inconvenience as well," he said.

"I hope that in due course there is a full explanation from the Commonwealth Bank."