National Australia Bank is seeking to raise around A$750 million through a series of notes backed by Australian prime residential mortgages, the first such transaction this year.Fitch Ratings and Standard & Poor's have both assigned their expected or preliminary ratings to the National RMBS Trust 2016-1, as follows: A$690 million Class A notes rated AAA(EXP)sf/AAA (sf) Outlook Stable A$45 million Class B notes rated AA-(EXP)/AA- (sf)       A$15 million Class C notes: not ratedS&P said its ratings reflected "the credit risk of the underlying collateral portfolio and the credit support provided to each class of notes." Credit support is provided by subordination and lenders' mortgage insurance cover for 10.4 per cent of the collateral portfolio, according to a presale media release from S&P.At the cut-off date, according to a Fitch presale report, the total collateral pool consisted of almost $751 million in loans underwritten by NAB. "The pool has weighted-average seasoning of 30 months, with a weighted-average unindexed current loan to value ratio of 58.7 per cent and a weighted-average current indexed loan to value ratio of 57.5 per cent," Fitch observedInvestment loans represent 23.5 per cent of the pool and interest-only loans represent 20.1 per cent, according to Fitch.Yesterday also saw the launch of a $250 million securitisation of nonconforming residential mortgages originated by specialist lender La Trobe La Trobe Financial Services.The arranger is Westpac Institutional Bank, also a joint lead manager, along with its counterparts at CBA and NAB. A statement from one of the bankers on the deal said it was expected to price on or before Wednesday 1 June.Eight tranches have been assessed by Standard & Poor's, starting with the AAA(sf)-rated $150 million Class A-1 tranche. These notes have "price guidance" of 170 to 180 basis points over one-month BBSW according to the JLMs.The next five tranches, from Class A-2a down to Class D notes, totalling approximately A$88 million at launch, are marked "not for sale" on the pre-sale advice from the JLMs. The notes have decreasing S&P preliminary ratings, from AAA(sf), down to BBB(sf), respectively.The class E and Class F notes, rated by S&P as BB(sf) and B+(sf), respectively, come with price guidance of 725 bps and 825 bps over 1-month BBSW. A further equity-type stake, comprising $4 million in seller's notes will be retained by the issuer.