An earnings boom in Commonwealth Bank's flagship retail banking arm has triggered predictions the division's profitability may be unsustainably high, as banks face pressure to put more emphasis on customers' interests over those of shareholders.

The standout performer in CBA's result this week , which was marred by a spike in regulatory costs, was the retail banking division led by the next chief executive of the banking giant, Matt Comyn.

The division includes CBA's vast consumer-facing bank, and its earnings swelled by 8 per cent to $2.7 billion, helped by a move to charge higher interest rates on property investor loans.

CBA chief Ian Narev said bank profitability should be looked at over the economic cycle. Credit:Bloomberg

But with regulators applying the blowtorch to the banking sector, analysts questioned how sustainable it was for CBA's biggest division to be making so much money from its customers.