Treasurer Scott Morrison has insisted the Government will return a surplus in 2020, despite ratings agencies casting doubt on the timeline and the debt ceiling being increased to $600 billion.

Credible budget return to surplus key points Key points: Estimated that budget deficit will be revised to $29.4 billion, before returning to $7.4 billion surplus four years later

Credit ratings agency Moody's casts doubt on predicted GDP growth, but Treasurer says forecasts are credible

Labor describes Treasury's wage growth figures as "optimistic"

According to Treasury estimates, the deficit will be revised up slightly to $29.4 billion next year before returning a $7.4 billion surplus four years later.

Mr Morrison has maintained this trajectory is credible and realistic — particularly given the Government has abandoned $13 billion in saving measures that were unlikely to pass the Senate — but credit ratings have other ideas.

The budget forecasts wage growth increasing from the current 2 per cent to 3.75 per cent, despite growth remaining at historically low levels.

While company profits are up — perhaps signalling some future wage growth — the budget projections have been described by some analysts as "sudden and unexpected".

In a statement, credit rating agency Moody's cast doubt on the predicted GDP growth and increasing revenue, saying those trends had failed to materialise in recent years.

"We continue to forecast a slower deficit consolidation than projected in the budget," Moody's associate managing director Marie Diron said.

"We estimate that GDP growth will be less revenue intensive than shown in the budget, consistent with somewhat lower tax buoyancy observed in recent years."

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Bur Mr Morrison said Treasury's growth forecasts were credible and less than what was predicted by the International Monetary Fund.

"They're smack bang in the middle of what the consensus forecasts are right across markets here in Australia," he said.

"We think they are conservative forecasts, they are forecasts that reflect currently what the mood is in the market."

Shadow treasurer Chris Bowen also questioned Treasury's wage growth figures saying they were "optimistic".

"Wage growth is at record lows in Australia and they are sort of assuming that it will all just whirr back to much more positive territory and they haven't explained how," he said.

Deficit ceiling increased, again

It was a small footnote hidden in the budget papers that revealed the Federal Government had increased its self-imposed debt ceiling by $100 billion to $600 billion.

The increase — forced by predictions that debt will reach $606 billion in four years' time — comes amid internal political pressure and ratings agencies questioning the timing of return to surplus.

The Coalition stuck a deal with the Greens to scrap the $300 billion cap in 2013 and replace it with a $500 billion cap, which former cabinet minister Eric Abetz now considers a "deeply regrettable mistake".

Mr Morrison said the ceiling had to be increased again to avoid key government services shutting down, saying "that's not a practical option".

The debt ceiling limits the amount of money the Government can own to invest in infrastructure or major projects (good debt), or to spend on recurrent spending such as welfare (bad debt).

Mr Morrison dismissed suggestions he would accuse Labor of living beyond their means if they increased the ceiling, saying that would be "very careless analysis".

"The Government will no longer be borrowing to pay for its everyday expenditure," he told AM.