Despite a softening economy the budget will be just short of a surplus in 2018-19 and record a $9.8bn surplus for 2019-20, according to Deloitte Access Economics.

In its latest budget monitor, released on Monday, Deloitte suggests the budget is improving due to booming company tax receipts even as crashing house prices and slow wage growth hit consumer spending.

Deloitte estimates the budget will be $3.1bn better off in 2018-19 and $5.7bn better off in 2019-20, compared with the official mid-year update in December.

The shadow treasurer, Chris Bowen, has seized on the Deloitte report, warning wages growth will underperform Coalition estimates and arguing the government has “no excuses” for adding $21bn in debt since Scott Morrison became leader.

Deloitte estimates that revenue will beat official forecasts by $2.9bn in 2018-19 and $3.7bn in 2019-20 due increases in company tax receipts of $2.3bn and $5.2bn. It credited increased coal and iron prices due to China’s stimulus and the dam wall tragedy in Brazil.

Deloitte says that revenues are “roaring” because “the only bits of the economy still moving at speed happen to be key drivers of revenues”.

“Yet that also means that revenues are vulnerable to any further signs of weakness,” it said.

Deloitte predicts that short-term positives will soon run out, with nominal GDP $4bn lower than official estimates in 2020-21 and $11bn – or 0.5% – behind by 2021-22.

“In particular, wage woes start to hurt: the gap between the treasury’s forecast of wages and [Deloitte’s] … grows steadily over time,” it said.

The treasury has predicted that wage growth will reach 2.5% by June then continue increasing to 3.5% by June 2021, while Deloitte predicts much slower growth of about 3% through 2021.

Nevertheless, surpluses are expected to grow to $15bn in 2020-21 and $20.6bn in 2021-22, with low unemployment driving savings in welfare.

Deloitte said that despite the 2018 income tax cut package, bracket creep will lift to $3.7bn in 2019-20 and to $9.9bn in 2021-22. Income tax as a proportion of pay will hit 20 cents in the dollar for the first time since 1999.

“That suggests we haven’t heard the last of tax cuts ahead of the coming election,” it said.

In the December mid-year economic and fiscal outlook, the government set aside $9.2bn for decisions taken but not yet announced, fuelling speculation it will further cut income taxes or bring forward the existing package of cuts.

Speaking to Guardian Australia in September, the treasurer, Josh Frydenberg, spruiked the benefits of the government’s three-stage plan which he said would see 10 million taxpayers pay less tax but did not rule out further income tax cuts.

Economic management and the return to surplus in 2019-20 are central to the Morrison government’s re-election pitch, along with the claim that Labor will impose $200bn of increased taxes.

Labor has framed the election as a “referendum on wages”, promising to boost wages growth while rebuilding the budget through closing tax loopholes in negative gearing, capital gains tax and ending tax refunds for excess franking credits.

Bowen accused the Liberals of mismanaging the economy “which is suffering from negative per capita growth, weak wages growth and flat productivity growth”.

He said the government is “irresponsibly riding on the coat tails of another temporary commodity price resurgence”.

Bowen noted that Deloitte was “particularly critical of the Liberals’ unrealistic wages forecasts” in the update.

“Labor is the only party with a plan to pay down the Liberals’ record debt and build the fiscal buffers we need while the economic circumstances allow.”