Very occasionally, the real world forces its way into the deliberations of federal policymakers, and when it does it’s a mildly shocking thing, like an open window bringing breeze and car horns and birdsong into a muzak-filled air-conditioned shopping mall.

This week it arrived in the form of Terese Edwards, from the National Council for Single Mothers and their Children, who was giving evidence to a Senate inquiry about a government plan to cut $4.8bn from family payments – which adds up to cuts in payments to a single parent with two children between 13 and 18 of about $3,000 a year, including axing a twice-a-year top-up payment.

The treasurer, Scott Morrison, quite correctly says the top-up payment was introduced because many families were finding themselves with a liability after being accidentally overpaid by the government because they had underestimated what their family earnings would be. Now that technology has overcome that problem, the “Christmas and July bonus payments have outlived their purpose”, the treasurer insists. The government also argues that once children are in their teens mothers can, and should, go back to work. Which, in an ideal world, is also true.

But these arguments ignore one big thing. For mothers who cannot find well-paid work, the current family payments – even with the bonuses – are barely enough to get by on.

Edwards explained that low-wage jobs and unemployment benefits left single parents with so little that they now relied on the “bonuses” for pretty important purposes – paying for any out-of-the-ordinary costs, such as car registration or replacing an appliance.

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She gave the results of a 2014 survey of single parents, in which 74% said they had trouble paying utility bills, almost 14% said utilities had been disconnected, 58% said their children had given up sport or other activities because they couldn’t afford the equipment or fees, 62% said they could not afford health or household insurance and 57% said they struggled to keep their car on the road.

And she read from stories the single mothers had sent her to pass on to the inquiry.

“My son never passes on school notes regarding anything that costs money. I’ve found them ripped up in his school bag, hidden in this room, etc. I think he has noticed my hands shaking so many times from monetary stress that he will do anything to keep things like this from me. He’s 13 and has desperately wanted to get a job since he was 11. He constantly asks why I hardly ever eat dinner but, fortunately, he still seems to believe that I absolutely love toast and two-minute noodles when I do eat.”

Or: “Please, please, please tell them ... all the pollies … that we will go over the edge if there are any more cuts. My luxury is our 18-year-old Corolla.”

Or: “Jack is great at sports, he pretended that he was injured so he would not get picked for the inter-school competition. It was $35. We just don’t have the spare cash.”

According to the government, the family benefit cuts – first announced in its disastrous 2014 budget and recently softened to try to win parliamentary approval – are the only way it can pay for a plan to spend an extra $3.5bn on childcare.

Treasurers have always had a thing for “offsetting savings” – focusing fellow ministers’ minds by demanding they look for ways to save as much money as they want to spend. Historically they have also used so-called “hypothecation” as cover for tax increases. The Medicare levy, for example, only pays for some of the costs of Medicare and is really just a tax increase, as was the 0.5% increase in the Medicare levy imposed by the Gillard government to “help” pay for the national disability insurance scheme (NDIS).

But it is more unusual for treasurers to suggest direct hypothecation of spending cuts – the fiction that the only possible way to find the money for one important thing is by cutting other specified type of spending. It’s a “we can only afford to buy a new car if we stop buying milk” type of logic, but it is indeed what the Abbott and Turnbull governments have been saying. They argue that a proposed $3.5bn childcare revamp can only possibly come from the pockets of the lowest-paid.

In fact, Morrison is suggesting hypothecation might be used to justify cutting welfare payments even further, because the social safety net won’t be sustainable once the government has to pay for the NDIS.

“While the government is fully committed to [the NDIS], the reality is that from 2019–20, the 0.5% increase in the Medicare levy will cover less than half of the commonwealth’s contribution,” he said this week, not mentioning that it was never designed to cover the full cost of the scheme.

Meanwhile, on a different day in a different parliamentary inquiry this week a different type of reality was dawning.

Chevron was explaining that reports it had paid just $248 in tax on an estimated $1.7bn Australian profit rerouted through the low-tax US state of Delaware were wrong. It had, in fact, paid nothing, because no tax was due – thanks to the company’s advantageously structured international borrowing. Neither Labor nor Liberal committee members were buying that explanation. Labor called it a “rort” and the Liberal senator said it was “immoral”.

BHP Billiton and Rio came under fire in the same committee for similarly convoluted and tax-minimising “marketing hubs”, which are under dispute with the tax office.

As former treasurer Wayne Swan said in parliament recently, the inquiry has “drawn attention to the legally questionable and ethically bankrupt tax practices of some Australia’s most senior corporate citizens” and that “tax avoidance on such a grand scale impoverishes us all” because there is not enough in the government’s coffers to pay for essential services.

But at the same time as leading corporate lights are defending practices that allow them to pay very little actual tax, the Business Council of Australia (Chevron, BHP and Rio are all members) and other groups are pressing for a corporate tax cut as part of what policymakers like to call tax reform. (They really mean tax “changes”, but change is so value-neutral – it can be good or bad – whereas “reform” sounds like it really has to be a positive thing.)

The government has said nothing definite, but a higher goods and services tax that pays for personal and company tax cuts seems likely.

But strangely, when it comes to this discussion, with multiple billions of dollars in revenue at stake, no one mentions hypothecation.

No one says, “Sure, we can manage a company tax cut to boost investment just as soon as major corporations start paying a reasonable amount of the tax they are already supposed to pay – but not until then.”

In fact in the last sitting period the government refused to pass its own legislation trying to force multinationals to pay more tax because Labor, the Greens and the crossbenchers in the Senate had banded together to insert into it a requirement that big private companies be required to reveal what they pay in tax. As I’ve argued before, that seemed an unwise place for the government to take a stand.

Both these bills will be before parliament in the final two sitting weeks of the year, which begin on Monday – the cuts to low-income families and single-parents’ benefits and the first small legislative step to try to force multinationals to pay a fair amount of tax.

And despite Scott Morrison’s determination to draw a thick black line from the promised extra payments on childcare to benefit cuts, rather than anywhere else in his budget, we all know that what the government can afford to spend is logically connected to everything else it spends money on, and all the revenue it receives.