Westpac has taken its place in the queue of Australian banks planning a covered bond issue, receiving ratings of Aaa and AAA on its US$20 billion program from Moody's Investors Service and Fitch Ratings.Fitch said the preliminary cover pool for Westpac consisted of 29,795 loans with a value of US$7.5 billion. The loan-to-valuation ratio of the pool is 62.7 per cent and the average seasoning is 2.9 years.ANZ, Commonwealth Bank and NAB all received credit ratings on their covered bond programs last week.While Australian banks market their debut issues one factor that will help them in their work over the coming months is the prospect that Canadian banks - which have been selling covered bonds with some gusto over the last four years - are beginning to hit their limits for this form of funding.The Canadian Financial Post reports that Canadian banks have already sold more than US$50 billion (A$48.4 billion) in covered bonds.In Canada, the regulator has capped covered bonds at four per cent of assets. In Australia, the cap is double that percentage.Research by ratings agency DBRS, quoted by the Post, found that Canada's CIBC bank had issued C$13.6 billion of covered bonds and was close to its four per cent ceiling.DBRS put the ratio for Canada's five other main banks at between 1.3 per cent (for Toronto Dominion Bank) and 2.8 per cent (for Royal Bank), though these ratios will have increased, taking into account a flurry of recent issuance.Canadian banks (which include one mutual credit union) have borrowed via covered bonds in Canadian dollars, US dollars, Australian dollars, euros and Swiss francs.