UBS has continued its forensic dissection of the Australian mortgage industry, moving on to analysis of data from Westpac, which the lender provided to the Royal Commission. UBS said that, for the first time, information on borrower's total debt-to-income ratios (rather than the often quoted loan-to-income) has been made available. These data pools were pitched as representative data from the bank, comprising 420 of Westpac's home loans, as analysed by PwC as part of APRA's recent review.APRA Chairman Wayne Byres said he found Westpac to be a "significant outlier", with PwC finding eight of the ten mortgage 'control objectives' were "ineffective".Martin North, principal of Digital Finance Analytics noted that Westpac's median debt-to-income ratio was calculated as 5.4 times, with 35 per cent of the sample having debt-to-income ratios greater than seven times. Further, 46 per cent of the mortgage applications had an assessed net income surplus of less than A$250 per week."This data raises questions over the quality of WBC's $400 billion mortgage book - accounting for 70 per cent of its loans," North wrote in a blog quoting the UBS research. "While WBC has undertaken significant work to improve its mortgage underwriting standards over the last 12 months, we expect it and the other majors to further sharpen underwriting standards, given the Royal Commission's concerns with responsible lending," North said. "This could potentially lead to a sharp reduction in credit availability."For its part, Westpac has reacted sharply to APRA's comments, and those of UBS, pointing out the review was initiated in 2016, whereby PwC examined a sample of 420 mortgage files. Since the review was delivered, Westpac said it could confirm that: it has reassessed the 38 loans that PwC believed would fail the standard using both the information on the credit files (which was available to PwC) and other information available to the bank at the time (which was not part of the PwC review)and on this basis all the loans would have been approved, apart from one loan (this loan is currently ahead of its repayments); only four of the loans in the sample are currently greater than 30 days past due, of which only one is greater than 90 days past due, well below the portfolio average for delinquencies; all of the loans in the sample were originated over 18 months ago and are now well matured; and of the original 420 loans, 90 have already been repaid/refinanced.Westpac CFO Peter King said: "Westpac's mortgage book continues to perform well as outlined in our most recent Pillar 3 disclosures for 31 December 2017. Our mortgage delinquencies and losses remain low both relative to historical and industry averages."At 31 December 2017 Westpac's mortgage 90+ day delinquencies in Australia were 0.67 per cent.Westpac said it would provide a further update of its mortgage portfolio quality at the group's interim 2018 results, expected on 7 May 2018.