Commonwealth Bank has the highest rate of growth of new mortgage lending among the big banks and is picking up the lion's share of property investor demand for finance, according to a review of the market by UBS banking analyst Jonathon Mott. The basis of the report, which was published this week, is that credit growth statistics miss the large flows into and out of banks' mortgage books. These flows help explain why banks are winning or losing share. The report said: "We believe there is a lack of understanding of what drives these outcomes. There is an assumption that the bank that writes the most new business delivers the highest credit growth. This is not necessarily the case. New fundings are the biggest moving part but [they are] only one factor."Mortgage flows can be broken down into new funding, redraws, interest and fees, pre-payments, property sales and external refinancing. The differences on the banks' mortgage flows have implications for their strategies and growth outlook."According to UBS, CBA's mortgage growth has been driven by new lending, particularly investment property lending. In the six months to June it wrote $34 billion of new mortgages - considerably more than its peers. Westpac wrote $24 billion of new loans in the six months to December, NAB wrote $23 billion over the same period, and ANZ $21 billion.UBS estimates that CBA is writing 28 per cent of all new loans. This compares with 20 per cent for Westpac, 19 per cent for NAB, and 18 per cent for ANZ.CBA has also had the highest rate of growth in new lending - up from $29 billion in the six months to December. The other banks have been relatively static in comparison.The report said: "Investors are rushing back into the housing market and have contributed more than half of the bounce in mortgage lending over the past six months. We estimate that CBA has picked up 35 per cent of this flow."The rapid improvement in CBA's new fundings relative to peers correlates with it becoming more aggressive on mortgage pricing. However, due to the small number of observations and a number of other factors we believe it is unfair to conclude that price was the only driver."However, CBA also had the highest level of repayments - $25 billion in the June half.UBS said NAB continues to gain share but not at the same rate as in 2011, when it "attacked" the mortgage market. "We believe that NAB is still seeing solid levels of new lending and an average run-off rate," the report said.Westpac's strength is its relatively low repayment run-offs, of $21 billion (in the December half). This is a result of higher levels of investment property lending and interest-only loans, which tend to stay on the books longer.Westpac's run-offs in the December half represent 14 per cent of its mortgage balances. This compares with ANZ's rate of 16 per cent, NAB's rate of 16 per cent and CBA's rate of 18 per cent.Westpac's weakness, however,