Westpac is achieving its goal of growing market share in lending markets in a sustainable way, chief executive Gail Kelly said yesterday.However, analysts at yesterday's interim financial results briefing wanted to explore the margin impact of the push for growth, putting questions about interest rate discounting, broker incentives and other competitive initiatives.Since the middle of last year, when Kelly announced that the bank was making a "tilt to growth", Westpac's mortgage portfolio growth rate has gone from 0.7 times system growth to 0.9 times system.At the end of March Westpac had A$338 billion of Australian mortgages on its books - an increase of five per cent over the previous corresponding period.Kelly said the aim was to get the growth rate up to system growth in the second half of the year.In other lending markets, Australian credit card share has increased from 22 per cent in December last year to 22.5 per cent now, and personal loan share has grown from around 26.8 per cent to 27.3 per cent over the same period.The bank has achieved these market share gains by increasing its "share of voice" with more advertising, adding more home lenders, beefing up its training programs and giving lenders more credit authority.Westpac chief financial officer Peter King said that the bank's asset spread (which is an element in the net interest margin) was down seven basis points over the March half.King said this was largely a result of contraction of mortgage spreads caused by competition and customers moving from variable rate loans to fixed (which have lower margins).He said that lower cost of funds over the same period offset most of the mortgage spread contraction. Kelly said she was comfortable with the decline in "front book" mortgage margins (the margin on new loans).On the business lending side, the loan book grew by five per cent.King said the margin outlook was neutral, with ongoing competitive pressure on lending being offset by lower funding costs.Kelly said: "Our focus on tilting to growth is delivering and this is expected to continue in the second half of the year."