CBA on Friday cut fixed rates for investors by between 0.25 and 0.5 percentage points across all terms, while it also cut some of its owner-occupier rates by 0.1 to 0.3 percentage points. A spokesman said the bank sought to deliver competitive products for its customers, "while maintaining our prudent lending standards."

Finance expert at Canstar, Steve Mickenbecker, said the rate cuts signalled a push into the property investor market for CBA, and rivals were likely to respond to CBA's rate cuts and its recent market share gains with their own pricing changes. "They've got to wrest some of that market share back from CBA - they cannot let that happen," he said.

CBA's strong growth in mortgages was a highlight of its profit result this week, with mortgage growth at 1.5 times the industry average. But the bank cautioned it was unlikely to continue growing at this pace, and there is also a concern among many bank-watchers that its share price is over-inflated. None of the 11 analysts covered by Bloomberg data have a "buy" recommendation on CBA and only three have a "hold."

Mr Johnson said CBA shares had risen because of a wider run-up in share prices, and he thought the bank had become "ridiculously expensive." "The market looks expensive and there are two really big stocks which people are afraid to sell, which are CBA and CSL," Mr Johnson said.

Morningstar analyst Nathan Zaia said CBA had benefited from other banks' "hiccups" in mortgage lending, but it was "only a matter of time until they [rivals] get things heading in the right direction again."