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.

Real wage growth has fallen into negative territory, relative to inflation, but the budget has forecast wage growth to rebound to 3.75 per cent by 2020, which would deliver a 16 per cent boost to tax receipts and help secure a surplus by 2021.

"The negative outlook on Australia reflects our view that if downside risks to government revenue materialise, then budget deficits could persist for several years, with little improvement, unless the Parliament implements more forceful fiscal policy decisions," the agency said.

To reach its projected surplus by 2020-21, S&P suggested in its statement, "additional restrictive measures" might be required to bring the budget under control.

It also took aim at the Senate's "unwillingness in recent years to legislate many of the government's fiscal policy measures, or doing so after considerable delay".

"This dynamic, which could continue, presents further downside risk to the outlook for fiscal balances," it said. "We therefore continue to think that budget surpluses could remain elusive beyond fiscal 2021."

Treasurer Scott Morrison has repeatedly highlighted about $25 billion in savings measures the government has passed through the Senate since the 2016 election to make the case that the government is able to work with the large crossbench and achieve fiscal consolidation.

S&P's judgment is broadly in line with Moody's, which last week also questioned the Turnbull government's real GDP and revenue projections over the next four years.

Before the budget, Labor shadow treasurer Chris Bowen said the Turnbull government should dump its company tax cuts which, if implemented in full, would forgo $65.4 billion in revenue over 10 years.

At the National Press Club on Wednesday, Mr Bowen said that, if elected, Labor would implement its changes to negative gearing, capital gains tax and superannuation to improve the structural balance.

"As Treasurer, I would be talking to the ratings agencies early in our term, talking them through our plans," he said. "They will be clear before the election. Whether we still have a AAA rating when Labor comes to office, if we're honoured with that mandate, is an open question."