The skirmishing began well in advance of the Budget. Take health, for example. Hospitals need capital investment, midwives demand better pay, people were promised free visits to the doctor, and more.
The moment the Budget speech begins, the opposition and an army of sceptical reporters go to work, looking for chinks in the government's armour.
National's new leader, Simon Bridges, and others will be out to convince us that the country needs a change of government and the Budget is the evidence.
Left-wing critics will find that not enough is being spent on most things and will accuse Labour of bowing down to neoliberal orthodoxy.
The 2018/19 Budget is only round one in a longer political boxing match.
Next year's Budget may be the crucial one, as the new government is planning substantial innovations to fiscal analysis.
The Child Poverty Reduction Bill, presently at the select committee stage, will require governments to set targets to reduce rates of child poverty and to report on progress.
It will amend the Public Finance Act to make governments explain how its annual Budget will contribute to meeting these targets. This brings a critical objective of social policy into the engine-room of fiscal policy and accountability.
The government's financial planning and the performance of the economy – and the interactions between those two domains – form the basis of Budget analysis and debate.
But the new government will introduce a wider framework for "sustainable inter-generational well-being". Financial/physical, social, environmental and human capital will be accounted for.
It should be possible, for instance, to estimate the capital value of clean rivers, based on the value of the "ecosystem services" that they supply. Then we could ask how effectively public spending and investment sustain or enhance such "natural capital".
This will have major consequences in future, as public finance and accountability will be calculated on a more holistic basis, beyond the traditional measures of economic productivity.
Mr Robertson is nonetheless bound by conservative fiscal-responsibility commitments, including budget surpluses and reduction of net public debt.
And he was pleased to announce that the surplus would be almost $1 billion higher than the Treasury had forecast in December.
Sounds fine, but we do not live in a risk-free world. Many economists have pointed to warning signs of another global meltdown. If that happens, the government would need to rethink its fiscal parameters.
By the time we get to the 2020 election, the people's verdict on Jacinda Ardern's government will depend a great deal on how well Grant Robertson performs on the next three Budget days.
* Associate Professor Grant Duncan teaches political theory and New Zealand politics at Massey University's Auckland campus.