According to Fitch's dinkum index, based on Australian RMBS transactions, mortgage arrears in Q4 2015 decreased 20 basis points, year-on-year, to 0.95 per cent. This was the lowest fourth quarter level for the past 11 years, which reflects the current low interest rate environment, strong property price growth and a strengthening of underwriting of mortgages from 2015 onwards, Fitch observed.Likewise, for mortgages that have blown out to more than 90 days in arrears, the rate remains very low at 0.40 per cent in 4Q 15. Fitch noted that factors such as strong house price growth, low unemployment, low standard variable rate and low CPI all contributed to benign conditions.In addition, tightening serviceability assessments recommended by the Australian Prudential Regulation Authority and the Australian Securities and Investments Commission are "positive the holders of new vintage RMBS transactions, particularly in the current low interest rate high house price environment," observed Fitch.In the current benign environment, Fitch suggested, most mortgage repayment delinquency may be due to non-economic factors - for example, divorce, illness or extraordinary expenses. In effect, the default rates and delinquency rates are probably as good as they can be, and borrowers have repaid loans to an amount equivalent to more than two years of scheduled repayments according to the RBA.Bucking the trend were self-employed borrowers, who have continue to experience financial difficulties despite positive serviceability factors, as indicated by the Low-doc Dinkum Index, which recorded a 32 basis point increase in 30-plus days arrears to 7.29 per cent in 4Q15.