Self-employed borrowers are falling behind with their mortgage repayments at a time of otherwise manageable repayment obligations for most borrowers.Fitch Ratings said its measure of mortgage arrears of 30 days or more showed an improvement of five basis points to 1.12 per cent during the June 2015 quarter.However, it found that "self-employed borrowers continue to experience financial difficulties despite the benign economic environment" with arrears of 30 days or more on low doc loans up 28 bps to 5.72 per cent over the quarter.Finance industry market researcher Canstar issued a ratings report on low doc loans last week - its first since 2009. When it last reviewed the category it looked at 109 loans offered by 30 lenders, but in the latest review it looked at 59 loans offered by 18 lenders.Canstar found that the variable rate on a low doc mortgage was, on average, 53 basis points higher than the average standard variable rate. On a three-year fixed rate loan, the average low doc rate was 41 bps higher than a standard loan.Canstar gave five stars to ANZ and Westpac for their fixed-rate low doc mortgages and five stars to AMO Group, Liberty Financial and Westpac for their variable rate low doc mortgages.Australian Prudential Regulation Authority data shows that low doc loans made up A$29.5 billion of the $1.3 trillion mortgage market in the June quarter (2.3 per cent of the total) - down from $34.8 billion (2.9 per cent of the total) in the June quarter last year.