Small lenders continued to erode the dominance of the major banks in new home lending in May, according to data collated by the country's largest mortgage aggregation network.The latest AFG Competition Index shows that CBA's monthly share of all new mortgage lending originated by the firm's affiliated brokers has almost halved in the last 12 months to 11.8 per cent compared to 20.5 per cent a year ago.While CBA remains the market leader in the first home buyer segment, with 18.1 per cent of loans written in May, it is now a laggard among the major banks for fixed rate and investment mortgages.The decline in CBA's monthly volumes coincided with the renewed push since June last year from the prudential regulator to cap lending to investment borrowers. "With CBA, AFG believes this the result of a deliberate strategy to pull back from investor and interest-only markets to meet the lending caps mandated by the Australian Prudential Regulation Authority," said AFG general manager, Mark Hewitt.Westpac remains the fastest growing mortgage lender, with its four banking brands accounting for more than 23 per cent of all mortgages written by AFG-tied brokers last month.Westpac's share is slightly down on a year ago when it accounted for slightly less than 25 per cent of total mortgages originated through the AFG network.While ANZ (15.8 per cent) and NAB (10.8 per cent) are continuing to increase market share, most of CBA's business is leaking to Suncorp and a string of small lenders.Suncorp has tripled its mortgage lending on a year ago, boosting its market share in May to 6.6 per cent from 1.9 per cent.The Brisbane-based bank was particularly aggressive on fixed rate mortgages in May, claiming more than a tenth of new loans in this segment of the market.Suncorp recorded its solid loan growth among AFG brokers across the country, including Victoria and South Australia.La Trobe Financial and AMP were among the other second-tier lenders to capitalise on CBA's abeyance in the last 12 months.Collectively, second tier lenders claimed 35 per cent of all mortgages sold by AFG brokers - up from 26.8 per cent share they recorded a year ago.