This would represent the lowest ROE for the Big Four since 2009 when average return fell to 13.5% as the global banking system was brought to its knees. ROE is a key measure of profitability for the banking sector and the expected decline shows how far the tide has turned for the sector with the Bank for International Settlements declaring only a year ago that Commonwealth Bank of Australia ( ASX: CBA ), National Australia Bank Ltd. ( ASX: NAB ), Australia and New Zealand Banking Group ( ASX: ANZ ) and Westpac Banking Corp ( ASX: WBC ) were the most profitable in the developed world. But APRA recently decided that our biggest mortgage lenders will have to lift their capital ratio by 2 percentage points to better protect our banking system from losses on home loans, which is estimated to force the Big Four into boosting capital levels by close to $30 billion.