Australian home buyers flocked to big offshore players such as HSBC, ING Groep NV, and Citigroup Inc., following a period of heightened regulatory scrutiny and customer backlash at the big domestic institutions. Those banks grew by 6.6%, 5% and 2.3% respectively, over the same period.

The loss of market share for the Big Four points to a challenging revenue growth environment at a time when their dependency on home loans has increased after selling scandal-hit businesses such as insurance and financial advice.

A string of scandals in the last two years has seen regulators pressuring the major banks to tighten their conduct and lending processes, thereby slowing down credit approvals.

Meanwhile, HSBC and Citi have invested in their relationships with brokers and sped up their loan approval processes. HSBC has also opened new branches.

"The big overseas banks have been less impacted by the Royal Commission," said Steve Mickenbecker, Financial Services executive at rate comparison site Canstar.

"Those institutions have been able to get the loan approvals through more quickly when they have been traffic jams at some of the bigger organisations, plus those banks have also been pricing aggressively in the last 12 to 18 months."

Commonwealth Bank, already Australia's largest, with one fifth of the mortgage market grew its home lending books by 1.2% in the six months to Dec. 31 to A$444.8 billion (231.44 billion pounds), according to Australian Prudential Regulation Authority (APRA) statistics.

The rest of its smaller domestic rivals - Westpac Banking Corp, National Australia Bank, and Australia and New Zealand Banking Group - shrunk their books by up to 1.6% each over the same period.

"The extent of ANZ, and now (Westpac), housing share losses are unprecedented," said Brian Johnson, a banking analyst at Jefferies in Sydney.

Due to the scandals and scathing revelations at a sector-wide misconduct inquiry, all but one of the Big Four have been forced to overhaul their management teams.

Commonwealth Bank, which reports first-half earnings on Feb. 12, was the first to see the early retirement of its CEO following a money-laundering scandal.

NAB, the third largest, lost its top executives after an industry inquiry into misconduct accused them of being unwilling to accept responsibility for the bank's misconduct.

Westpac, the second largest with a 23% share of the mortgage market, is searching for a new chief executive following the resignation of its CEO over a child exploitation payments scandal.

Investment conglomerate Macquarie Group, which was largely unscathed by the inquiry, grew its mortgage book at a dramatic rate of 14% over the same period - albeit from a much lower base of about 2.5% market share.

By Paulina Duran