Global rating agency Standard & Poor \'s (S&P) said on Friday it had downgraded Australia\'s big four banks\' credit ratings as a result of changes to its criteria for assessing banks. Westpac Banking Corporation, Commonwealth Bank of Australia, ANZ Banking Group and National Australia Bank all had their issuer credit ratings cut by one notch from AA to AA minus. The big four now have the fourth highest rating on the scale of Standard & Poor\'s. Australia\'s major investment bank Macquarie Group had its long- term rating downgraded from A minus to BBB, but it still has the ninth highest rating on the S&P scale. Earlier this week, S&P downgraded the ratings on 37 world\'s largest banks as the rating agency tightened its risk assessment criteria, prompted by turmoil on global financial markets. S&P says the new criteria will allow it to change its ratings more quickly when it sees threats to bank funding or if it sees governments become less willing to bail out creditors.