When the Commonwealth Bank delivers its annual profit next month, dividend-hungry investors will be watching even more closely than usual.

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That's because there is growing debate about the big banks' ability to reward their shareholders with ever higher dividends.

And since CBA is the biggest bank in the country, it tends to set the tone for Westpac, National Australia Bank, and ANZ Bank.

The big four are rightly seen as dividend-paying machines. Between them, they pay about $20 billion a year to shareholders. As the graphic shows, this amount has grown strongly during the past few years.

Lately, however, questions are being asked about whether this cash splurge can continue growing at such a pace.

You see, the banks have been singled out for tougher treatment by financial regulators who are concerned about the risks in the home loan market, which is the driving force behind the banks' profits.

To make the banks safer if there were a housing crash, the regulator last week said it would require them to hold larger buffers for absorbing losses if loans are not repaid.

That sounds incredibly technical, but it means the big four will have boost their buffer by $12 billion over the next year.

The buffer, known as "capital", will come from some combination of higher prices for customers, asset sales, skinnier dividends than would have been the case, or capital raisings. And that's probably not the end of it, as last week's move was an "interim" measure.

Governments overseas are busily making similar changes – and our lenders need to keep up because they borrow tens of billions from foreigners every year.

It is hard to quantify how much more capital banks will need, but it is pretty clear last week's changes are only the beginning. This steady rise in capital requirements could make it harder to keep on churning out higher dividends.

Banks are loathe to actually cut their dividend in absolute terms unless they really have to – such as when their loan losses are going up sharply. This is what happened during the global financial crisis, but we're not in a situation like that now.

Nonetheless, several experts reckon there will be little growth in bank dividends from here on in.

Morgan Stanley analyst Richard Wiles has recently forecast a period of "flat" dividend growth for investors in the banks.

Watermark Funds Management investment analyst Omkar Joshi​ said he expects softer or no growth in the banks' highly prized dividends, because banks will be retaining more of their profits.

Indeed, it's already happening. NAB is keeping its dividend unchanged this year, and Westpac made the smallest increase in five years in its first half.

@clancyyeates