Westpac surprised credit markets yesterday by announcing a minimum A$750 million prime mortgage-backed bond that will be sold via the Series 2012-1 WST Trust. Westpac has been the only major bank to use the RMBS market for funding over the last nine months.Westpac's last issue was via the Series 2011-3 WST Trust and was in October last year. The simple three-tranche structure is the same as the one being offered now and, ultimately, raised A$1.6 billion.No indicative pricing has been given for this current issue, but the A$690 million in Class A notes (with a three-year weighted average life) in Westpac's last transaction were priced at 125 basis points over 30-day bank bills. Westpac should be very pleased if this margin could be achieved again.  The RMBS market has struggled in 2012, being overshadowed by covered bond issuance from the major banks and the very wide credit spreads the banks were prepared to pay to entice investors. The credit spreads on covered bonds have tightened considerably in secondary market trading but now provide a new benchmark for 'riskier' RMBS.