Commonwealth Bank's New Zealand operations generated big profits in the first half of the financial year, thanks to a sharp, 28 basis-point rise in net interest margins, which increased to 1.92 per cent, from 1.64 per cent, in the June half.Its New Zealand operations include ASB and Sovereign insurance. ASB's net profit rebounded to NZ$283 million in the six-month period to December 2010 from a loss of NZ$10 million in the same period of 2009. While the last year's figure was distorted by payments in the conduit tax dispute, the fall in impairment provisions and the improvement in interest margins explain most of the latest result.A continued shift to variable rate home loans, from fixed rates, and repricing of business loans and other products supported the lift in margins.CBA's CEO, Ralph Norris, said that he expected a further improvement over the next half, "but nowhere near as dramatic as the improvement we've seen in this period-on-period."The strong result, however, masks the deterioration in asset quality in the June 2010 quarter. Impairments increased NZ$37 million in the latest half compared with a drop of NZ$1 million in the September 2010 quarter.As with the business in Australia, ASB lost market share in home loans and business loans, and also in deposits over the last year.ASB said its home loan market share fell 60 basis points over the year before, to 22.4 per cent.Market share in business loans has fallen 10 basis points since December 2009. On Tuesday, Bank of New Zealand said its market share had grown in home loans, agribusiness and deposits.