The Turnbull government will need to show supreme budget heroics next week to explain why the deficit has increased by billions of dollars in the past five months, Deloitte Access Economics says.

The economic consultancy firm has released its respected Budget Monitor, outlining the pressures on federal government finances before Malcolm Turnbull’s government brings down its first budget on 3 May.

It warned the electorate was at risk of “sleepwalking through another election campaign” in which no major party faces up to the fact that spending growth is far outpacing revenue growth, unless the government used the budget to be honest with voters.

Commonwealth deficits to 2018-19 were likely to be $21bn worse than official forecasts just five months ago, it said.

It called on the Treasury and the finance department to use the budget to promote believable forecasts about the rate of economic growth, and the price of key commodities such as iron ore and coal, so voters had a better understanding of the real costs of the services they wanted.

“Major miners, who account for a sixth of company tax in an average year, have taken huge profit hits,” the Budget Monitor said. “Overall profit taxes are forecast to fall $4.7bn shy of the latest official estimates for 2015-16.

“[And] special mention needs to go to resource rent taxes, which continue to head the way of the dodo. Low oil and gas prices seem to be lingering longer than a great aunt at Christmas, with commensurate carnage in collections.

“And spare a thought for superannuation taxes, which are suffering from further downward revisions after a horror run in recent times.”

The report said the budget boom of the past decade had turned into a budget bust, with the combination of China’s slowdown, low commodity prices and weak wage growth cutting overall revenue by $4.1bn in 2015-16, with a further shortfall of $3.5bn in 2016-17.

It projected a Commonwealth deficit of $41.7bn in 2015-16, saying this was “a substantial $4.3bn worse than projected” in the mid-year budget update in December.

“Back in 2013 we didn’t get a Pre-Election Fiscal Outlook. What got served up to the public was more of a Pixies, Elves and Fairies outlook,” the report’s authors said.

“2016 is an election year. That’s a big reason to hope the 2016 [budget] won’t be like the last Pre-Election and Fiscal Outlook. The 2013 PEFO essentially assured voters that within a decade, whoever they voted in would deliver over $80 billion a year in as yet unidentified savings on spending and that much of the revenue shortfall of earlier years was locked in.

“Err, that’d be ‘no’ and ‘no’ respectively.”

The Deloitte Access report made a dramatic downward revision to official GDP forecasts, saying GDP growth was likely to fall from 2.8% in 2015-16 to just 2% in 2016-17.

It forecast an increase to 2.8% and 2.7% in 2017-18 and 2018-19 respectively.

Bureau of Statistics figures show annual economic growth is currently 3%.

Deloitte director Chris Richardson said Treasury and Finance could choose to “play safe” in this budget by failing to make its revenue and spending forecasts more realistic.

“But if they do, the Australian electorate will once again be surprised when the federal budget remains just as broken as today,” he said.