Westpac more than doubled the size of its first mortgage-backed bond since December 2009, to A$1 billion, on Friday.The AAA-rated tranche, in the Series 2011-1 WST Trust, comprising 92 per cent of notes, was priced at 100 basis points over one-month swap. The pricing of Class B and Class C notes consisting of 3.4 per cent and 4.6 per cent of notes was not disclosed.The pool backing the deal is a closed one, and the weighted average loan-to-value ratio indicated for the issue size, of A$435 million, at launch was 64.4 per cent.Westpac's previous RMBS, issued in December 2009, saw the top-rated $1.84 billion tranche priced at a spread of 130 basis points over one-month swap.