Australia's AAA credit rating has been confirmed by ratings agency Standard and Poor's, but the agency has left the nation on negative watch and questioned whether the Turnbull government will achieve a surplus by 2020-21.

In a decision that will be seized on by the Coalition – which was sweating on the AAA rating before last week's federal budget – S&P joined the other two major agencies, Moody's and Fitch, which had already affirmed the rating following the budget.

However, S&P said the nation's economic outlook would remain negative because of the "risks to Australia's fiscal consolidation plan and risks to the economic, fiscal, and financial stability outlook should the rapid growth of credit and house prices continue".

In particular, the agency has flagged concern about whether projected revenues – for example, from income tax revenue that comes from wages – will materialise.