For the past six months, a Senate inquiry has travelled across the country to find out why more than 1,600 bank branches have shut across regional and rural Australia in the last six years.

On Wednesday, the inquiry heard directly from those in charge of the closures — the bosses of Australia's "Big Four" banks: Commonwealth Bank, Westpac, NAB and ANZ.

The CEOs defended their decisions to fold the branches, saying it was the result of weaker foot traffic and a clear shift towards digital banking.

But that wasn't all they were grilled over — and their appearances gave us an insight into the future of cash in the economy, the digital boom, and how they plan to offer their services to customers right around the country as the transition continues.

Here are the key takeaways from their day under the microscope.

Cash is no longer king (and hasn't been for a while)

Commonwealth Bank CEO Matt Comyn, who was the first to appear before the committee, noted the shift away from cash transactions was undeniable.

"Five years ago, 43 per cent of all point of sale transactions were cash. Today the figure is around 15 per cent," he said in his opening statement.

"And yet every week customers transact more than $18 billion through the CommBank app — an increase of 64 per cent in just two years."

But that doesn't mean we're rapidly approaching the end of the line for our notes and coins, either. Mr Comyn told the hearing the bank — which is Australia's largest — won't move away from cash.

"We certainly have no plans to remove cash distribution or the provision of cash in Australia," he said.

"I don't think that that's feasible, and I don't think that would be desirable, certainly in the foreseeable future."

The cost of maintaining cash, however, is an expensive exercise.

"We estimate that continuing to support distribution and availability of cash cost CBA approximately $400 million each year, which works out to be roughly $40 for every one of our 10 million customers," he said.

"Many of our customers don't use cash though, and these customers cross subsidise those to do."

Digital transactions are booming

The move away from cash has come with a dramatic shift towards the digital services offered by the big four.

On Wednesday, every CEO told the hearing just how dominant digital transactions are for their banks:

Matt Comyn, Commonwealth Bank: "Our mobile banking app [has] more than 8 million users. On average, they log in 39 times per month. It's clearly one of the most important, if not the most important feature that customers use."

Peter King, Westpac: "96 per cent of Westpac customer transactions are now digital. Customers who only use a branch represent around 3 per cent of our 13 million customers."

Ross McEwan, NAB: "93 per cent of interactions with our personal customers occur through digital channels ... only 3 per cent of our personal banking customers do their banking exclusively through a branch, and we're now processing more than 1 billion payments online annually."

Shayne Elliott, ANZ: "More than 90 per cent of [aged over 65] customers use at least one self-service option to do their banking, such as our ANZ app, internet banking or ATMs ... while most customers prefer digital channels for many of their transactions, branches continue to be important."

Senators spoke of the need for better infrastructure in the most remote parts of the country, including internet access and mobile phone reception, to ensure those digital services would be consistently accessed, which all four bosses agreed on.

"The provision of digital infrastructure and services is extremely important," Mr Comyn told the hearing.

Less foot traffic is one reason for the closures

All four bank bosses told the hearing that the majority of branches they have closed have been as a result of reduced foot traffic, or customers coming through the doors, but other factors were also in play.

Mr Elliott from ANZ told the hearing most of its in-branch clients are businesses, not individuals.

"Branches used to be a place where consumers would go — mums and dads, families, individuals — a lot," he said.

"Today, that's really not the case. It's increasingly become a place for small businesses."

The shift towards digital banking has also had an impact, with Mr Elliott noting that research from the University of South Australia showed older Australians were shifting online.

"They found that most older people like and accept the online banking environment and that Australians over 65 are responding in line with younger cohorts in adopting digital banking," he said.

NAB's Mr McEwan said its decision making process in closing branches was linked to its overall activity.

"It's the drop off [in foot traffic] and customer usage of the branch network," he said.

"It's not just purely transactional. It will take a number of factors, but it is around the activity of the branch.

"There comes a point where not enough are coming into a branch to actually make their service viable and particularly as their are other services they can avail themselves on.

"That is just one indicator ... that we look at when we're making a broader decision around a branch."

But NAB was forced to admit that their planned closing of a branch in Ocean Grove came despite there being an increase in the volume of its transactions in the past three years, after persistent questioning by Liberal senator Gerard Rennick.

"That contradicts a lot of your prior information, though, that you said the reason why you were closing branches was because the number of transactions were decreasing, and yet we've got an example here where they are actually increasing," Senator Rennick said.

"I have an issue with the quality of information that you're providing to this inquiry and whether you're genuinely acting in the best interests of the Australian people who underwrite your social licence, but take that as a comment."

Most banks have paused closing regional branches for now

Since the inquiry began in February, Westpac and ANZ have committed to suspending bank closures, while the Commonwealth Bank has agreed to keep them open until at least 2026.

"We've made it very clear we're committed to staying in regional Australia until 2026," Mr Comyn said.

But NAB is sticking to its guns, and has shut around 30 regional branches in the past six months — a point Senator Rennick bluntly questioned Mr McEwan about.