NAB says it does not currently possess the technical means to stop its digital payment services being used as an ersatz messaging platform to stalk and abuse customers.
The issue of harassing and abusive messages in low-value transaction descriptions was revealed by CBA in early June, which uncovered over 8000 instances in a three-month period.
The 'Big 4' banks have since said they are weighing technical countermeasures to counter the behaviour.
However, NAB’s group chief executive officer Ross McEwan told a parliamentary committee last Friday that the bank's digital payment platforms are currently too complex and unwieldy to effectively filter all abusive messages out.
McEwan said NAB needed to find a way to block abusive transaction descriptions "across multiple payment structures".
“You can probably do one, but you can't do the probably 50 that come in and out of the organisation through different pipes," he said.
"We need find a solution across those to block them out, and we don't have that at the moment."
The Australian Banking Association has formed a working group with the 'Big 4' banks to implement an industry-wide approach.
But with the threat of regulation looming, individual banks have started dabbling with measures to block abuse using keyword filters.
Westpac chief executive officer Peter King told the same parliamentary inquiry committee that the bank flags about 150 items for review each month, though it was reluctant to disclose details for fear of tipping off abusers.
He said that the bank monitors messages for words, including “kill”, “maim” and “control”, but that abusers were finding ways to circumvent filters.
“We're looking straight out for those words, but people also find creative ways to spell them without using the exact letters, so we look at both the simple and more complex words,” King said.
At a previous hearing, ANZ chief executive officer Shayne Elliott said he was aware of the problem but that the bank was grappling with some privacy concerns.
“The difficulty here is what can be done about it? It's not an easy thing for us to manage, because I'm not sure it's appropriate that we monitor customers' communications,” Elliott said.
However, ANZ’s Australia retail and commercial banking group executive Mark Hand said the bank had taken steps to block payments containing foul language.
“We will block payments where the message contains swear words. That's just a computer monitoring of what goes on," Hand said.
"There are keywords that will identify and block that payment. So, the customer will make that payment a different way or reprocess that payment without that type of message. We can't stop all of them, but that's one measure that we have put in place."
Some of the banks have updated their terms and conditions to afford themselves legal protections to cancel abusive transactions or accounts.
However, the parliamentary committee has also sought that banks ventilate their views on the issue of who takes responsibility for involving police.
“We don't at the moment and it's rather difficult to implement real-time monitoring of every transaction that goes through," CBA's deputy CEO David Cohen said.
"From our point of view, we're starting off on the basis that, first of all, we want to stop the abuse, where we can.
"Secondly, we want to stop those customers being customers if they perpetuate it. [Police referral] is probably something that we would have to look at - it's just the practicality of doing that."