Westpac is set to make the largest buyback of government guaranteed debt to date in the local market, after announcing an offer to buy back the entire A$3.6 billion of March 19, 2012 notes.Westpac is offering a margin of 10 basis points over the bank bill rate.The notes were first issued in March 2009, for $3.06 billion, when Westpac sold floating and fixed rate tranches. The fixed rate tranche was sold at a coupon of four per cent, while the floating rate notes were issued at a spread of 60 basis points over swap.