MELBOURNE (Standard&Poor's) Feb. 13, 2013--Standard&Poor's Ratings Services today assigned its 'BBB' issue credit rating to the proposed convertible preference share (CPS) issue by National Australia Bank Ltd. (NAB; AA-/Stable/A-1+). NAB has said that it intends to use the CPS offer for general corporate purposes, and that after the CPS is issued it will qualify as Additional Tier 1 capital for regulatory capital purposes. Standard&Poor's rates the NAB CPS three notches below NAB's stand-alone credit profile of 'a'. The issue rating reflects: The securities' risk of subordination; The risk of partial or untimely payment; and A contingency clause requiring mandatory conversion into common equity on the activation of a nonviability trigger. We have assessed the