The former Liberal leader John Hewson has issued a warning to the treasurer, Scott Morrison, as he puts together his mid-year budget review.

“The fact we are going to lose the triple-A credit rating is a foregone conclusion, it’s just a question of timing,” Hewson told Sky News on Sunday.

The professor of economics believes the government is just muddling through and is a long way off getting a deliverable path to surplus by the end of the decade.

“I don’t see any realistic debate by either side … to deliver that sort of outcome,” he said.

Morrison will hand down his mid-year economic and fiscal outlook on 19 December.

A recent independent analysis anticipates the budget will have blown out by about $24bn over the next four years, partly due to record low wages growth hitting tax revenues.

The shadow treasurer, Chris Bowen, said the government was persisting with measures that will never get through the parliament – the so-called zombie measures – while ignoring structural changes like Labor’s plan to reform negative gearing.

The government is adamant that altering negative gearing would be harmful to the economy, a view it took to the last election, but fellow Liberals NSW premier Mike Baird and planning minister Rob Stokes have said they now believe it is time for a review of the concession.

“The government just looks ridiculous,” Bowen told Sky News.

“They should adopt a sensible approach [for the next election] … I don’t hold out much hope, but they should.”

Liberal frontbencher Arthur Sinodinos said neither wages or corporate tax revenues had grown strongly since the economy reaped the benefits of the mining investment boom.

“What we’ve got to do on the one hand is continue to promote strong growth in the economy,” Sinodinos told ABC television.

That was why the government was pursuing a 10-year plan to cut company taxes, which would reduce the cost of doing business and lead to higher jobs growth, he said.

The former Labor frontbencher Craig Emerson said the problem both sides of politics had faced in recent years was the way budget forecasts for the four-year estimates were put together.

The first two years are forecasts made by Treasury and the third and fourth years are technical assumptions.

“You can’t get to surplus on assumptions and this is what has dogged both sides of politics all the way through from about 2012 onwards,” Emerson told Sky News.