The Reserve Bank says the Australian economy will grow far more slowly than forecast in Tony Abbott's May budget, opening up the possibility of a multibillion-dollar hole in Malcolm Turnbull's first budget, due next May.
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The revenue hit is estimated to be somewhere between $3 billion and $11 billion.
The revised Reserve Bank forecast comes on top of news that the Treasury itself plans to downgrade its longer-term projections for growth, potentially slicing billions more from projected revenues.
It comes after Treasurer Scott Morrison declared that Australia had "a spending problem, not a revenue problem" and said that none of any extra goods and services tax raised as a result of tax changes would be used to boost the overall tax take.
The RBA has revised down its forecast for economic growth this financial year to an average of 2.25 per cent, a full half a percentage point below the budget forecast of 2.75 per cent.
Sensitivity analysis in the May budget indicated that a cut of half that size would add $5.6 billion to next year's deficit if it was brought about by lower export prices.
Deloitte partner Chris Richardson said if it was brought about by other means, the hit to the budget would be lower, perhaps $3 billion.
"It is definitely material," he said. "There's a revenue problem as well as a spending problem."
The Australian Taxation Office revealed this week that net tax collections for 2014-15 were $9.4 billion down on what was budgeted. Preliminary Finance Department figures for the first three months of this financial year show revenue on track.
The RBA said commodity prices had slipped 2 per cent in the past three months and growth in China was slowing.
Uncertainty about China and the outlook for steel remained "a key risk for commodity prices, particularly prices for iron ore and coal".
Mining investment would continue to slide as existing projects finished and there was "little, if any, evidence" of a pickup in non-mining investment.
Inflation would end the financial year at around 2 per cent - the bottom of the RBA's target band and a full half a percentage point below the budget forecast.
The RBA said its forecasts were constructed using the "technical assumption" that it would cut interest rates one more time before May as expected by financial markets.
"Lower growth forecasts, lower inflation forecasts and a lower cash rate assumption," said ANZ economist Felicity Emmett. "The [Reserve] is edging closer to cutting rates."
The Reserve Bank played down its forecast downgrade, saying that, overall, its forecasts were "little changed from August". But the August growth forecast was itself 0.25 points below the budget forecast, meaning its forecast is now 0.50 below the budget's.
Treasury secretary John Fraser said on Thursday that the Treasury would also downgrade its long-term growth projections, in part because the population was growing more slowly than had been expected.
It would outline the rationale in the next few weeks and would publish the updated forecasts in December.
The charts in the budget that predict a return to surplus by 2019-20 were built around an assumption of fast economic growth of 3.5 per cent in the five years to 2021-22.
The most recent growth figure was just 2 per cent, for the year to June. The Reserve Bank is now forecasting only a slight lift, to 2.25 per cent.
"Labor knew and said that the budget projections were optimistic," shadow treasurer Chris Bowen said.
"It's time the government released the tax green paper so the Australian people can see exactly how hard low and middle income earners will be hit by an increase in the GST."
