Mr Morrison's warning comes against a back drop of low wages growth and stagnant company tax receipts, which have held back government revenue and stoked fears a cut to the AAA rating is increasingly likely.

There is a view in the Coalition government that the agencies, as a senior source put, are "itching" to downgrade Australia's rating - a move that would hurt, forcing up borrowing costs for federal and state governments, and have a flow-on effect on household mortgages.

Ratings agency Standard and Poors put Australia's rating on negative watch in July, a clear sign it was most likely to cut the rating first.

The warning from the Treasurer on the AAA, and on the surplus, came after Deloitte Access Economics' Budget Monitor predicted a $24 billion blowout in the deficit over the next four years.

The government is hopeful it will secure passage of its superannuation changes, raising more than $5.2 billion, and a crackdown on rorting of VET FEE help loans, which would potentially save billions more and send a clear signal to the three major agencies before the end of the year that the government is capable of delivering budget repair despite its tiny majority.

Mr Morrison told Fairfax Media that Parliament passing unlegislated savings measures before the mid-year budget update on December 19 would "send a strong signal to the ratings agencies that the Australian Parliament is committed to restoring the budget to balance".

"S&P have made it clear that parliamentary support for implementing the government's fiscal consolidation plan will be a key factor in their decision to maintain Australia's AAA credit rating," he said.

"The [cost of the] corporate tax cut component over the budget and forward estimates, the whole package, so from turnover zero all the way through, is around $4.9 billion. But they're blocking savings of around $19 billion."

Earlier on Monday, the Treasurer said it was self-evident that as revenue projections moved around, the budget's return to surplus timeline would change.

When asked about his pledge to return the budget to balance by 2021, he softened the ground for delay.

"What I said was that the projections would return to balance in 2021".

"I've always been careful about that and that's why I pull you up on that. I know others in the past have made bold predictions and promises about this. The 2021 budget was a prediction based on the numbers. The numbers move. They always have," he said.

If the "numbers aren't there", the return-to-surplus timeline would not be met.

"That's simple arithmetic."

But Labor finance spokesman Jim Chalmers said the Deloitte Access projection of a $24 billion write-down in revenue would undoubtedly put further pressure on Australia's AAA credit rating.

"If Australia loses its AAA credit rating, Scott Morrison and Malcolm Turnbull will have nobody to blame but themselves," he said.

"What we've had is the deficit for 2015-16 blow out by eight times what was projected at the 2013 election. We've had a 2016-17 deficit triple and we've had a blowout in net debt of more than $100 billion since the Abbott-Turnbull government was elected. With a record like that, no wonder the ratings agencies are ringing the alarm bells about Australia's debt and deficit."

As the final two sitting weeks for 2016 began, debate got under way on the Registered Organisations bill, one of two industrial relations bill along with re-establishing the Australian Building and Construction Commission to crack down on union bad behaviour. The government is confident it will be able to pass these bills with crossbench support

Finance Minister Mathias Cormann confirmed that, as previously reported, the government's 'priority' company tax cuts would be delayed until next year, as would proposed changes to paid parental leave.

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