To return to surplus and to do it while avoiding growth in tax collections would require a literally unbelievable degree of spending restraint. Remember, though it gets little notice, Abbott is also promising to impose new taxes, increase taxes and eliminate tax breaks. These are partly to help cover the cost of the taxes he's getting rid of and partly to help pay for his new spending promises. He's proposing a 1.5 per cent levy on big companies to cover the net additional cost of his paid parental leave scheme and a 0.5 percentage-point increase in all rates of income tax (aka the Medicare levy) to help cover the cost of the national disability insurance scheme (both raising $16 billion over four years). To help cover the cost of abolishing the mining tax he's proposing to save $4.7 billion over four years by cutting business tax breaks: ending the instant asset write-off, removing accelerated depreciation for motor vehicles, ending the phase-down of interest withholding tax on financial institutions and ending the ''tax loss carry-back''. Also to help cover the cost of abolishing the mining tax he proposes to save $3.7 billion in four years by effectively increasing the superannuation contributions tax for those earning up to $37,000 a year, and save $1.6 billion in four years on no-longer-forgone super tax breaks by delaying for two years phase-up in compulsory employer contributions.

And all this is before we get to Labor's as-yet-unlegislated tax rises, which Abbott has quietly indicated he would proceed with: extra revenue of almost $10 billion over four years from measures to ''protect the corporate tax base'', cut research tax breaks, increase cigarette tax and impose a levy on savings accounts. When you see the list of tax hikes that accompany Abbott's grand tax-cutting gesture, it doesn't exactly inspire confidence he could keep taxes down in a way none of his predecessors has managed to. And when you realise that - according to the earlier reckoning of Saul Eslake, of Bank of America Merrill Lynch - his election promises involve extra government spending of almost $15 billion over four years, it doesn't inspire confidence he could achieve the herculean spending restraint needed to get the budget back to surplus as well as keep taxes down. The medium-term projections in Treasury's pre-election economic and fiscal outlook, about which I suspect we'll be hearing a lot more after the election, demonstrate how challenging the budget task will be in the coming decade. According to the projections, if the government elected this Saturday sticks to Labor's strategy of limiting average real growth in spending to 2 per cent a year and not allowing tax collections to exceed 23.7 per cent of gross domestic product, the budget surplus will recover to 1 per cent of GDP by 2020-21.

But, since spending is projected to grow at an underlying real rate of 3.5 per cent a year, this would require an unprecedented restraint. And, even so, it would still require tax collections to grow 1.5 percentage points faster than the economy - equivalent to an ultimate $26 billion a year in today's dollars - over the decade. Alternatively, were spending allowed to grow at its underlying rate, this could still leave us with a growing surplus, provided unrestrained bracket creep was allowed to cause tax collections to grow 3.3 percentage points (an ultimate $56 billion a year) faster than the economy. Loading And, get this. Were you to let spending grow at its ''natural'' rate, but limit growth in tax collections to a ceiling of 23.7 per cent of GDP the rest of the coming decade beyond 2018-19 would see an ever-rising budget deficit. Whoever wins this election, I'll be amazed if taxes do anything but keep rising.