The Coalition might also be having problems with its pledge that its costings will be done by the Parliamentary Budget Office. According to The Australian Financial Review, the PBO has costed Opposition Leader Tony Abbott's maternity leave scheme at more than $5 billion a year once it's up and running, or $14 billion over its first three years of operation. Nonetheless, such challenges are sent to be fiddled with – by, for example, excluding public servants from the scheme.

About the only real difference between the PEFO and ES is that the PEFO authors include an alternative view of what the unemployment rate might be in the “projection” years of 2015-16 and 2016-17. The ES and the first bit of the PEFO make the rather heroic assumption that unemployment will suddenly drop from 6.25 per cent at the end of June 2015 to 5 per cent 12 months later. You don't need the computer modelling grunt of Treasury to think such rapid employment growth would be extraordinary – and that the RBA would react to an economy growing that quickly by boosting interest rates to cool it.

The more credible PEFO “alternative projection” is a more gradual closing of the output gap with unemployment easing to 6 per cent at the end of 2016 and 5.75 per cent in 2017 – which would add $3.2 billion in extra unemployment benefits over those two years. Both sides will no doubt prefer to ignore that alternative rather than pluck another $3.2 billion out of the budget air.

The authors signing off on PEFO, Treasury's Martin Parkinson and Finance's David Tune, also go to some effort to explain the difficulty of forecasting animals as wild and unpredictable as the Australian and global economies.

“Against the backdrop of a still challenging global outlook, the Australian economy is expected to transition away from resource investment-led growth towards broader based growth, although this transition may not occur as smoothly as forecast,” the document warns. In other words, forecasting is a mug's game and even Treasury's best efforts need to be taken, not with outright disbelief, but with some understanding of the craft's limitations.