The federal budget is now due to return to the black a year earlier than expected, in 2019-20, but that wafer thin surplus wouldn't exist if it wasn't for $84 billion in "off-budget" investments made by the Turnbull and Abbott governments.

It's not that the $84 billion isn't mentioned in the budget at all but, as independent economist Saul Eslake explained, it only appears under the misleadingly named "headline" deficit/surplus figure, which doesn't get much public attention.

"That counts towards the headline deficit, but not to the underlying budget deficit, which is the principal focus of both the Government's own commentary and the analysis that's done by outside observers of the budget," he told The Business.

Highlighting the lack of attention these figures receive, Mr Eslake only noticed the scale of such spending days after the budget lock-up, as he went through the finer details in hundreds of pages of Treasury papers.

But the former ANZ chief economist and member of the Parliamentary Budget Office's panel of expert advisors isn't the only one paying attention.

Labor MP Julian Hill is deputy chair of the Parliament's Joint Committee of Public Accounts and Audit and is deeply concerned by the size of this non-transparent spending.

"You see $55 billion of equity investments off-budget and, on top of that, there's another $29 billion of concessional loans, contingent liabilities," he told The Business.

"That totals $84 billion of off-budget financing in some form, which is an enormous figure over five years and deserves closer scrutiny."

The reason this spending, which was $27.5 billion last financial year but is expected to fall to $20.7 billion next year, isn't counted in the underlying budget balance is that it is "financial asset investment".

As the name implies, the Government sees this funding as an investment that will generate a return — not spending going out the door never to come back.

However, it's still money that has gone from Government coffers that must be paid for through revenue (mainly taxes) or debt.

'Fiscal timebomb' won't blow up AAA credit rating

But Mr Eslake said, like any investments, they carry risks if future returns don't live up to expectations.

"Well it could be that the investments which the Government is making in these corporations at some point in the future have to be written off or written down," he warned.

However, due to accounting practices virtually unique to the Australian Government, a future write-down in one of these investments or loans wouldn't even appear in the budget's cash balance in the year it's made.

"A write-down of existing financial asset investments would not impact the underlying cash balance," noted the Finance Minister Mathias Cormann in a written response to ABC questions.

"Any write-down of concessional loans, such as HELP, and fair value movements in equity investments are reported as an 'other economic flow' in the operating statement."

Mr Hill said there are a couple of reasons that write-downs might happen.

"If they're not going to make a commercial return, above the rate of borrowing or if the cost of capital is not going to be repaid in a reasonable timeframe, then they're effectively a fiscal timebomb," he cautioned.

This timebomb wouldn't affect Australia's credit rating though — the only major agency that currently has Australia's AAA on a negative outlook is S&P Global Ratings, and its local analyst, Anthony Walker, said he looks past the number that gets all the media attention.

"Our fiscal and debt assessment is based on the Government's headline cash balance, opposed to the underlying cash balance, and includes any investments in financial assets such as the NBN and Snowy Hydro," he told ABC News.

"Changes to the accounting treatment of these investments would have no effect on our treatment. Our assessment also includes growing infrastructure spending of Australia's state governments."

Around 40pc of infrastructure spending now 'off-budget'

Mr Eslake also shies away from labelling the investments a timebomb, but he is concerned about the rapid growth in this form of capital spending.

"Over the five years to 2021-22, over 40 per cent of the Government's total capital expenditures will be financed in this way — that is through investments in or loans to Government-owned corporations — as opposed to about 30 per cent over the last five years, which includes much of the expenditure on constructing the National Broadband Network, and less than 10 per cent in the five years prior to that," he observed.

Finance Minister Mathias Cormann was not available for an interview but sent a detailed response to emailed questions. ( ABC News: Matt Roberts )

Senator Cormann said the growth in this form of financing is due to the Government's increasing commitment to infrastructure building.

"In recent years the Government has started to use a wider range of methods to support infrastructure, seeking to take a more active role in managing its significant investments in infrastructure and partnering with state and territory governments and the private sector to deliver infrastructure," the minister noted in an email.

"This has included significant investments in financial assets."

Mr Eslake said the financial implications are not just potential future asset value write-downs but growing losses within these Government-owned corporations.

"The net operating deficit (operating revenue minus operating expenses, not including capex) has widened from less than $1 billion a year before 2012-13, to $4 billion in 2016-17, $5 billion in 2017-18 and a forecast $6 billion in 2018-19," he noted.

'Cooking the books for future generations'

So where is all this money going?

Aside from the NBN, which Mr Hill said has soaked up almost $50 billion, the Deputy Prime Minister's office sent a list of current and future transport projects being funded at least partially through Government equity or concessional loans.

Sunshine Coast Airport, Sydney's Moorebank freight transfer terminal and the Adelaide to Tarcoola rail upgrade will all receive hundreds of millions of dollars via these means.

The New South Wales Government received a $2 billion concessional loan for Sydney's WestConnex motorway.

But the two biggest recipients of Federal Government transport investment will be the Western Sydney Airport, to the tune of $5.3 billion, and the Melbourne to Brisbane Inland Rail project, which is receiving a $9 billion equity injection.

It is the last, and biggest, of those investments that has really raised eyebrows.

"I can't find anyone who thinks you can make a profit off building a rail line," Mr Hill said.

"So this growing popularity of this method of financing infrastructure does raise questions when you have a look at the Inland Rail, the questions over the NBN, which are growing, about whether its really cooking the books for future generations."

However, Senator Cormann maintains the Inland Rail line is a commercial venture.

"The 2015 business case for Inland Rail is publicly available and was independently assessed by Infrastructure Australia in 2016," he responded.

"Inland Rail is a long-term infrastructure asset, with an economic life well in excess of 50 years.

"The accounting classification of the Commonwealth's contributions to the Australian Rail Track Corporation (ARTC) for the delivery of Inland Rail as equity is on the basis that the Commonwealth expects to earn a real return (i.e. above the long-term inflation target) from ARTC over the life of the investment."

However, even with construction costs assumed to rise at 2.5 per cent for the first five years while revenue inflation is expected to be 2.7 per cent (above the midpoint of the RBA's target range), the Government's own business case by PwC found Inland Rail did not deliver a commercial return.

"On a net present [value] basis Inland Rail's ongoing benefits do not outweigh the large capital investment required upfront over a 50-year operating term," it noted.

"However, once operational, Inland Rail runs profitably (i.e. revenue more than covers ongoing operations and maintenance costs), the NPV of which is approximately $2.2 billion over 50 years.

The Government's own business case shows the Inland Rail won't break even until the 2060s. ( Supplied: ARTC )

"Overall, Inland Rail is not NPV positive over the assessment life and thus requires government investment to deliver the infrastructure that would support the service offering."

Investment financing 'can be perfectly appropriate'

Not that Mr Hill is critical of all financial asset investment, saying it is not a question of whether such funding should be cut back, but rather whether it is being properly assessed.

"Where something is a productive investment that'll make a commercial return and repay its equity, then it can be perfectly appropriate to leverage Australia's balance sheet," he said.

Then again, he cannot be too critical without being accused of hypocrisy.

Labor controversially funded its National Broadband Plan using this model, and the NBN accounts for much of the rise in off-budget capital spending over the past five years.

But Mr Hill argues that the NBN has now well and truly become the Government's baby.

"They've changed the NBN model, it's their NBN, they've committed $29.5 billion by way of an equity injection and another $19 billion by way of a concessional loan," he said.

"So it's up to the Government to account and tell everyone that they're confident the NBN is worth what they claim it is."

You can see more on this story on The Business at 9:45pm (AEST) on ABC News Channel.