Households with two incomes are spending an average of 29.3 per cent of their income on mortgage payments - up from 28.2 per cent a year ago.Moody's Investors Service said current low mortgage interest rates had failed to offset the impact of rising house prices over the past year."The implementation of interest rate hikes this month will further increase delinquency and default risks for mortgages," the ratings agency said.In Sydney, where house prices rose more than any other Australian capital over the past year, households are spending an average of 39.2 per cent of their income on mortgage repayments - up from 36.1 per cent a year ago. It is the highest level since 2001.Housing affordability also deteriorated in Melbourne, where households spend 32.1 per cent of income on mortgage repayments, and Adelaide (21.7 per cent of income).Affordability improved in Brisbane (23.3 per cent of income) and Perth (21 per cent).