S&P Global Ratings downgraded the credit scores of more than 20 smaller financial institutions as it warned the risks of a property market downturn are increasing, although expectations of government support saw the very largest banks spared.

About 23 issuers had their ratings lowered, including Bank of Queensland, Bendigo & Adelaide Bank and AMP Bank.

The credit assessor exempted the Big Four banks - Australia & New Zealand Banking Group, Commonwealth Bank of Australia, Westpac Banking Corp and National Australia Bank - on the assumption that the government would step in to provide support if needed.

It also opted not to lower the ratings for Macquarie Group and its banking unit.