Housing affordability in Australia has deteriorated over the past year, according to Moody's Investors Service.Moody's said that during the 12 months to March households spent 27.6 per cent of their monthly income on mortgage repayments. This was an increase from 27 per cent during the 12 months to March 2015.Moody's said a fall in the average house price in the March quarter might signal a peak in repayment costs.Another factor that will improve affordability is this week's decision by the Reserve Bank to lower the cash rate by 25 basis points.Sydney is the least affordable city, with households spending 35.6 per cent of their income on mortgage repayments during the year to March.Households in Melbourne spent 30 per cent of their income on mortgage repayments over the some period, compared with 27.2 per cent the previous year. Households in Adelaide spent 23.2 per cent of their income on mortgage repayment (up from 21.9 per cent), and in Brisbane they spent 24.3 per cent (up from 23.6 per cent).Affordability improved in Perth - down from 22.6 per cent to 21.5 per cent.