Lower interest rates supported strong growth in new lending activity during the September quarter, but this was offset by a higher level of loan repayments, Commonwealth Bank reported in its September quarter update yesterday.CBA made a profit of around A$2.1 billion in the three months to September (unaudited cash earnings) - a 14 per cent increase over the previous corresponding period.The bank provided few details, saying that revenue grew ahead of costs; that the group's net interest margin was down "marginally"; and that household deposit growth was strong.Mortgage lending figures published by the Australian Prudential Regulation Authority last week show that CBA's mortgage book grew by 1.3 per cent in the three months to September, compared with system growth of 1.2 per cent.The bank said its ratio of customer deposits to total funding rose from 63 per cent in June to 64 per cent in the September quarter. According to APRA figures, CBA's household deposit book grew by 3.2 per cent in the three months to September.A highlight was the loan impairment expense: the ratio of impairment expenses to gross loans fell from 17 basis points in the June quarter to 16 bps in the September quarter.Home loan and credit card arrears have fallen since June.