Jam Tomorrow was the moniker given to former Finance Minister Michael Cullen's Budget as he withheld money for treats for taxpayers and it seems this Government is set to pick up where he left off.

Prime Minister Jacinda Ardern and Finance Minister Grant Robertson stood up yesterday and announced Labour would not be able to afford to do what it had wanted to do in next month's Budget because it had to put its money into health and education.

The reason for this, they insisted, was because National had left things in a more parlous state than Labour had expected.

Such claims should be taken with a grain of salt.

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Labour's plight is due to more than such matters - it also has the costs of its support partners' policies to contend with.

But the saga of the leaking buildings at Middlemore Hospital could not have come at a better time for an exercise in blaming the other side.

Ardern pointed to Middlemore as "emblematic of what we are seeing across the board" and it would release further examples of National's legacy over the next six weeks.

These sniper attacks on National's record were necessary "to make our case" for the steps required in the Budget.

She insisted the reason she and Robertson were talking about it was an attempt to "manage expectations" and a "transparency" exercise.

Translated: it was an attempt to soften up the public for what is coming (or, rather, not coming) with the added benefit of an excuse for a prolonged attack on the previous Government's record.

While Ardern was willing to be more than transparent about the problems National had apparently left it with, she was less willing to be transparent about the things Labour would not be able to do as a consequence.

National finance spokeswoman Amy Adams accused Labour of "crying wolf", saying it had plenty of money thanks to National but its own spending promises were too big.

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Labour has admittedly found itself in a Catch-22 situation partly of its own making.

During the campaign Labour announced it would stick to fiscal responsibility rules to stay in surplus and pay down debt.

It also ruled out lifting income tax or imposing new taxes in its first term which now means it has no way to raise the revenue it needs to pay for all its promises.

Both were necessary sacrifices to ensure it gave itself a chance to get into government.

In that regard, it worked. But now payday has arrived and the kitty is short - just as many had warned.

Despite the apparent urgent need for spending, Robertson and Ardern are sticking to the fiscal responsibility plans.

They believe with some justification they will be the barometer by which Labour is judged on its economic management.

But Budget 2018 is not the ultimate target of the softening up exercise on voters.

The ultimate target is 2020 when Labour will go back to the polls and seek to convince the public of the need for new or higher taxes.

Yesterday saw Labour start to make its case for that.