So where is the extra $15.1b to come from?
Nearly half of it ($7.4b) comes from a higher revenue track, 85 per cent of which is the result of scrapping the tax threshold adjustments that Finance Minister Steven Joyce announced in May and which are scheduled to kick in on April 1 next year. In addition, Labour is counting on getting $700m more than the present Government from cracking down on multi-national tax avoidance.
Most of the rest ($7.2b) of the higher committed spending is scooped out of a line item in the Budget called the operating allowance. This is the provision for new spending (or revenue-reducing) initiatives in future Budgets.
The Prefu includes a cumulative $11.7b of that unallocated spending. Labour's fiscal plan reduces it to $4.5b altogether over the next three Budgets.
That is pretty tight, especially if the wish lists of coalition or support partners have to be accommodated. But it is worth remembering that in the wake of the global financial crisis, we had a couple of zero Budgets, when there was no such allowance: in effect, Bill English told the public sector there would be no new money for them and any new initiatives would have to be funded by "reprioritising" within their existing budgets.
In this election season, National has so far only nibbled at its $11.7b kitty. Its proposed operating initiatives (as opposed to capital items like roads, a new stadium for Christchurch and a new hospital in Dunedin) add up to less than $1b, including provision for subsidised GP visits and an extension to paid parental leave.
Assuming, perhaps generously, that Labour can live with the tighter unallocated operating allowances its fiscal plan includes, its budgeted operating expenditure over the four years to June 2021 would be just $8b, or 2.3 per cent, higher than National plans.
Its revenue would be $7.4b, or 1.6 per cent higher. That does not look like an increase under the weight of which the economy would crumple.
Ah, but that does not take into account any changes arising from Labour's planned review of the tax system, its critics would say.
It is, of course, a bit rich of National to be scornful of the plan for a tax working group, when that is exactly what it did in 2009.
And if you think the tax system needs some significant changes - and it does - then it makes sense to be a bit careful about how to go about it.
The tax working group approach of picking all the best brains and publishing the advice from officials, academics and tax practitioners ahead of any final report and political decisions is simply good process.
It should not be subverted by shoving microphones in front of Jacinda Ardern and asking her to rule out this or that and, if she does not, going "Aha! That must be the sinister secret plan! Labour must come clean."
It is unfortunate - even if politically expedient - that Labour has limited the review's options by excluding a new top income tax bracket, or excluding the family home from a capital gains tax. A case could be made for including it, given that owner-occupiers enjoy the benefit of imputed rents.
It is axiomatic that the broader the base, the lower rates can be, and New Zealand's tax base is too narrow.
In any case, changes to the tax system would have to be accommodated within the fiscal rules Labour and the Greens have agreed.
One of those Budget responsibility rules is to manage spending to keep it around the historical trend level of 30 per cent of gross domestic product. In the fiscal plan released this week, it averages just under 29 per cent over the next four years.
Another of the rules is to run a surplus in the operating balance excluding gains and losses, across an economic cycle. Together, those objectives would imply keeping revenue broadly stable as a share of GDP across the cycle as well.
That suggests changes to the tax system would be broadly revenue neutral, a matter of redistributing the tax burden - hopefully to something fairer and less distortionary - rather than increasing it relative to the size of the economy.
