Commonwealth Bank of Australia launched a A$1.6 billion residential mortgage backed securitisation transaction yesterday. This is CBA's first public RMBS issue for 2018, and such is the demand, with most of the notes already allocated, it is likely to be finalised today.The Medallion Trust Series 2018-1 is a seven tranche RMBS transaction, backed by prime first ranking Australian mortgages originated by CBA. In this regard it is the same as the two previous Medallion transactions, both issued in 2017.The top-ranked A1 notes - accounting for $1.5 billion of the $1.63bn total face value of the notes to be issued - have been "preplaced" according to the CBA, and there is no ability for the transaction to be upsized. Where the deal differs from previous Medallion issues, and indeed other Australian RMBS deals, is that it has a three-year revolving period where principal collections can be used to purchase new receivables subject to portfolio parameters, eligibility criteria and substitution criteria being satisfied, noted NAB corporate finance director, Ken Hanton, in his weekly "Securitisation Insights" letter to clients.Hanton expected the deal would be priced today (21 September).The "indicative" pool of loans backing the issue will be entirely comprised of "Full Doc" loans, of which almost 59 per cent will have a weighted average loan to value ratio of 32 months seasoning; and 15.0 per cent of the portfolio is supported by lenders mortgage insurance provided by Genworth.The arranger was CBA, and was also one of the joint lead managers and book runners, alongside Deutsche Bank.