National Australia Bank Ltd chairman Michael Chaney says the financial institution is in "very strong" shape and a downgrade of the company's credit rating is inappropriate.
Ratings agency Moody's Investors Service has downgraded its outlook on NAB's credit rating to negative, while Standard & Poor's has put the bank on a negative credit watch, warning it has a one-in-three chance of being downgraded.
The move by the ratings agencies comes after the bank revealed an $830 million writedown on assets related to US residential mortgages.
"We certainly don't believe any downgrade would be appropriate," Mr Chaney told ABC Television.
"The bank's in very strong shape and at an operating level things are going extremely well, but that's a matter for us to sit down and talk to the ratings agencies with, and to assure them that's the case."
By contrast, Standard & Poor's has the other major banks on a stable outlook, including the ANZ Banking Group Ltd even though that lender announced much heftier $1.2 billion credit writedown for its second half.
Some banking analysts have warned they are expecting further provisioning against another $4.5 billion in collateralised debt obligations (CDOs) that NAB has as a potential US recession damages the value of the underlying loan assets.
Mr Chaney also painted a bleak outlook for the global credit crisis.
"It's possible the situation will get quite a bit worse," he said.