Australian business banking was an underperformer for ANZ during the March half, with the business held back by weak loan growth, pressure on corporate debt margins and a couple of big impairment charges.Corporate and business banking contributed A$546 million of the $1.5 billion of cash profit reported by the Australian division. The corporate and business banking result was seven per cent down on the previous corresponding period and unchanged from the September half.There was no growth in corporate and business banking revenue over 12 months.The corporate banking group was responsible for a large increase in impairment charges. The bank's chief financial officer, Shayne Elliott, said the increase, from $13 million in March last year to $70 million in the latest half, was due to two corporate exposures. These debts contributed to a 23 per cent increase in the credit impairment charge for corporate and business banking.The corporate loan book contracted by two per cent from the previous corresponding period, the Esanda finance company book was unchanged, the regional business loan book grew five per cent and the business banking (middle market book) grew by one per cent.The one bright spot was small business banking, whose loan book increased by 16 per cent.ANZ has made a big push in the small business market. In March the bank announced that it would allocate an additional $2 billion in lending to small business over the next year.Elliott said the bank expected to see its investment in the small business segment produce more growth in assets. He said there was an increase in mergers and acquisitions activity, which would provide some growth in the middle market.And he said deposit margins were improving.