Standard & Poor's on Friday said it had downgraded the credit ratings of the big four Australian banks as part of changes to the way it assesses risk.

Westpac Banking Corporation, Commonwealth Bank of Australia, ANZ Banking Group and National Australia Bank all had their issuer credit ratings downgraded by one notch from AA to AA minus.

The Macquarie Group had its long-term rating downgraded from A minus to BBB.

The move, widely expected, follows S&P saying it was revisiting ratings on 37 of the world's largest banks as part of "applying its new ratings criteria for banks", prompted by turmoil on global financial markets.

Earlier in the week, it downgraded the ratings of major US banks, including Citigroup, Goldman Sachs, Wells Fargo, JPMorgan Chase, Morgan Stanley and Bank of America.

The changes reassess risk associated with investment banking, how banks are funded, how liquid banks are and their capital standards.

Part of the rationale for the move was to allow more room for banks to improve their ratings, if their finances were deemed more secure.

Commonwealth group treasurer Lyn Cobley said the bank remained one of the few banks globally in the AA category.

"Since the onset of the financial crisis in 2008, the group has increased its capital base and improved its funding and liquidity position by increasing customer deposits and long term wholesale debt and reducing our use of short term wholesale funding," she said in a statement.

"At this point we do not expect this to have any material impact on our funding plans or expected pricing of our new issuance."

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