Westpac will pay 140 basis points over the one-month bank bill swap rate on the A$1.06 billion top tranche of its latest issue of residential mortgage-backed securities.The Class A notes of Series 2012-1 WST Trust have a weighted average life of three years and AAA ratings are expected from Standard and Poor's and from Moody'sPricing on the $39.1 million of Class B notes and the $52.9 million of Class C notes was not disclosed. The bank said the Class A notes would benefit from eight per cent subordination provided by the Class B and C notes.The issue is backed by a pool of prime residential mortgages, which have a weighted average loan to valuation ratio of 62.6 per cent and a weighted average seasoning of more than three years; there are no low-doc loans.Westpac is the first of the big banks to issue RMBS this year.When ME Bank issued RMBS in April it paid 150 basis points over swap for an AAA-rated tranche with a weighted average life of 2.8 years.And when ING Direct issued RMBS in March it paid 145 basis points over swap for an AAA-rated tranche with a weighted average life of 3.2 years.