Fitch revised the outlook on CBA and ASB's "long-term issuer default rating" (a measure of the likelihood an institution can repay its creditors) from stable to negative. That means that the rating agency is considering downgrading the banks' credit ratings in the future.
Roche argues that because ASB Bank would rely on its parent to bail it out if it got into difficulties, if CBA is deemed less able to pay its debts, then ASB's creditworthiness is also impacted. But the same could happen the other way around too.
"If ASB's parent remediates its operations according to the plan in the report, that should strengthen the rating, and we would expect to look to revise it back to stable."
ASB didn't want to comment on the downgrade but pointed out in a statement to the stock exchange that its rating with the two other main agencies remains unchanged.
"Standard & Poor's long-term issuer credit rating is currently AA- with a negative outlook and Moody's Investor services long-term debt rating is A1 with a stable outlook."
Massey University banking expert, Professor David Tripe said, in theory, a negative outlook rating might make it more expensive for ASB Bank to borrow money on the wholesale market. Basically, the higher the risk of default, the higher the interest rate a lender might charge.
However, given that both CBA and ASB have high ratings to start with (AA-), lenders might not be too concerned, Tripe said.
"It's not going to be a big deal. If they were further down the rating scale there might be a bigger impact, but the difference between a rating change and the impact on the cost of funds isn't linear."
The next possible blot on the bank rating horizon will be the Australian Royal Commission report into Misconduct in the Banking, Superannuation and Financial Services Industry, Roche said. The interim report is due in September, with the final document expected early next year.
However, unless CBA's performance comes out as being significantly worse than that of other Australian banks, it might not affect ratings, which reflect comparative performance, Roche said.
"The base case is you still have a reasonable operating environment for banks in Australia."