Australia's second largest credit union, the Adelaide-based Australian Central, is getting ready to launch its maiden prime residential mortgage-backed securities deal with a two tranche $350 million offering, to be issued through Light Trust No. 1.The profile of the loan pool is pretty benign, with an average loan size of $136,000 and average loan seasoning of 29 months, which may be some sort of record for this market.Standard variable rate loans account for 39.5 per cent, with fully verified loan documentation at one hundred per cent, with the portfolio consisting of no line of credit and a tiny proportion of interest-only loans.