Growth in housing finance for residential property investors continues to outpace growth in finance for owner-occupiers - a trend that will strengthen the Reserve Bank's view that the composition of the housing and mortgage markets has become unbalanced.According to the latest RBA lending data, lenders' owner-occupier mortgage books grew by 0.4 per cent in August, compared with the previous month. The month-on-month growth rate has not changed since May.Owner-occupier mortgage balances have grown by 5.4 per cent over the 12 months to August.Investor loan portfolios grew by 0.8 per cent in August, compared with the previous month, and by 9.2 per cent over the 12 months to August. The annual growth rate has increased from 5.9 per cent in August last year.The A$471.1 billion of outstanding investor mortgages make up 33.9 per cent of total mortgage balances.In its September quarter Financial Stability Review, published last week, the Reserve Bank said the low interest rate environment and strong price competition among lenders had translated into a strong pick-up in growth in lending for investor housing - noticeably more so than for owner-occupier housing or businesses. "Recent housing price growth seems to have encouraged further investor activity," the RBA said."As a result, the composition of housing and mortgage markets is becoming unbalanced, with new lending to investors being out of proportion to rental housing's share of the housing stock."Lending to investors is expanding at a fast pace, which could be funding additional speculative activity in the housing market and encourage other (more marginal) borrowers to increase debt," it said.The RBA said it was talking to the Australian Prudential Regulation Authority and other members of the Council of Financial Regulators about additional steps that might be taken to reinforce sound lending practices, particularly for lending to investors.