As per my post below, here is the direct quote from Standard and Poor's issued a few days ago, which explains how their very strong AA- credit ratings for all four major banks--and CBA in this case--rests crucially on their assumption that these are taxpyayer-backed institutions (ie, will be bailed out by the government if there are ever funding problems):
"Our counterparty credit rating on CBA is two notches higher than the [stand-alone credit rating], reflecting our view of a high likelihood of extraordinary government support in a crisis. This reflects our view of CBA's high systemic importance in Australia, and our assessment of the Australian government as highly supportive of institutions core to the national economy."
