The CBA did not separate out the margins of its various subsidiaries - other than to say Bankwest's improved by one basis point, or 0.01 per cent in the past six months - but its overall margin was 2.29 per cent, up from 1.99 per cent a year earlier. Mr Zumbo, an outspoken critic of the approval of the deal that allowed the CBA to buy Bankwest, said yesterday's result was proof Australian consumers were "at the mercy" of the Big Four banks. "The evidence is clear - you look at net interest margins before 2008, when there was an independent Bankwest and St George and the likes of Aussie Home Loans, Wizard and RAMS (all since bought out by the CBA or Westpac), and they had been heading downhill for years," he said. "As soon as those buyouts occurred, the margins are heading up again. Competition was the only reason they fell. The lack of competition has seen them creep up." Much of the increase was due to raising interest rates above the Reserve Bank's official cash rate rises.