Mortgage lender FirstMac Ltd has had to pay a slightly higher margin on its latest issue of residential mortgage-backed securities. FirstMac priced the top tranche of its latest deal at 115 basis points over the bank bill swap rate. This compares with pricing of 110 bps over swap in June.On Friday, the mortgage funder priced the FirstMac Mortgage Funding Trust 2E-2013, raising A$400 million. It was a dual currency issue, made up of A$255 million and £85 million.Pricing on the $22 million A1 tranche was 115 bps over swap. The £85 million A2 tranche was a soft bullet, which means that no principal will be re-paid prior to the re-financing date. Pricing on this tranche was not disclosed.Pricing on the $20.8 million AB tranche, the $9.6 million B1 tranche and the $1.6 million B2 tranche was not disclosed.FirstMac's chief financial officer, James Austin, said in a statement that the higher pricing on the top tranche was a result of the large amount of RMBS issued this year.Austin said: "At almost $30 billion, 2013 RMBS issuance is almost double the volume in 2012."He said there were 20 investors in the issue - 15 of them were real money accounts.The overflow of supply in mortgage-backed bonds may explain the revival in use of covered bonds by big banks, where the pricing is better.Last week, Westpac sold A$900 million of 10-year covered bonds, priced at 85 basis points over swap. This is only the second issue of covered bonds this year, with ANZ having also sold 10-year covered bonds in August. ANZ paid 100 bps over swap.