The August earnings season saw the Commonwealth Bank of Australia report yet another full-year record profit of $8.6 billion, ANZ banked $5.2 billion in the nine months to June and National Australia Bank lifted third-quarter profit by 7 per cent to $1.7 billion.

ANZ, NAB and Westpac's financial year ends in September. Westpac does not give quarterly updates.

Despite these pretty impressive numbers, the financials sector has been trading relatively flat for the month of August, up just 0.1 per cent. CBA is down 2.9 per cent, although it has traded ex-dividend this month, while ANZ and NAB have fallen 0.2 per cent and 0.3 per cent, respectively. Westpac has bucked the trend, rising 1.2 per cent.

"I think the market is a little concerned about whether this is as good as it gets for these guys," Philo Capital head of listed equities Hugh Dive said.

"Against the background of relatively benign credit growth, it's difficult to see when the price-to-earnings ratio is justified."

Uncertainty surrounding the Murray Financial Services Inquiry and the impact that it may have on capital requirements and the ability to pay increased dividends in the near term has also been hanging over the banks.

The big four are also facing increased competition for business lending from cheaper overseas lenders, Mr Dive said.

"Property trusts have traditionally been big borrowers. Ten years ago they would have had all their debt with the big banks, now they're increasingly sourcing that money offshore because they can get it at a much longer tenure and at pretty attractive rates," Mr Dive said.

The search for yield and low earnings-risk have buoyed Australia's big banks over the past three years and made them the biggest drivers on the Australian sharemarket in that period.

"You've got to remember where in the cycle we are with the banks," Perpetual head of equities Matt Sherwood said.

At a time when unemployment is at a 12-year high and economic growth is below trend, earnings risk is increasing for banks, he said.

"What's really supporting their earnings is that credit growth is ok, bad and doubtful debts are low, their margins are solid and I tend to think that a lot of people are beginning to suspect some of those factors will be less supportive moving forward."