Over most of the past year Westpac has aimed for a big increase in the Australian mortgage business and was able to increase share from 25 per cent in March 2009 to 27 per cent at the end of the latest half. The mortgage book grew 18 per cent to $277 billion in the past year.Westpac's mortgage business has grown at 1.6 times system over the past year. Chief executive Gail Kelly said she expected it to drop back to growth in line with system for the rest of the year. Kelly said the opportunity to add that much share through organic growth was a "once in a decade event".But the cost of that acquisition has been high. The mortgage spread has fallen 12 basis points, as the bank has had to fund its increased lending with high=cost term deposits and wholesale money.This contraction in the spread contributed to a decline in the bank's net interest margin of 11 basis points since September to 2.28 per cent.Kelly rebutted suggestions that the bank was likely to dip below system growth or that it was now discounting (off its elevated price point) in order to maintain lending growth recently.The bank disclosed for the first time earnings from the Rams Home Loan brand, bought in late 2008. Rams contributed $3.9 million to cash earnings and generated five per cent of mortgage growth.Kelly said the bank was pursuing a long-term strategy. Having picked up 355,000 home loan borrowers, including 60,000 first home buyers, it was looking forward to increasing its share of wallet among that expanded customer group.Kelly is betting that Westpac can hold that share in the face of greater competition from smaller rivals and that it can achieve ambitious cross-sell targets. Typically, a Westpac mortgage customer has three of the bank's products. Twenty-three per cent of the bank's customers have four products or more.Another bet Kelly is making is by loading up on mortgages at a time when the Australian consumer is making up a larger proportion of the bank's impairment charges.Delinquencies in the Australian consumer portfolio rose in the March half, which the bank put down to rising interest rates, and the delayed impact of rising unemployment in 2009.Credit card delinquencies (payments 90 days past due) jumped at the end of last year and are at their highest level for three years - 1.19 per cent of the portfolio.Mortgage delinquencies rose three basis points to 35 basis points. The bank considers this level sound. However, the number of mortgage customers in the bank's hardship assistance program has doubled since September.