EXPLAINER: Depending on who you ask, and when you ask them, the Government either has no money for the upcoming Budget or all it could ever need.

The Government has spent the last few weeks on a down-play tour, dousing hopes it was going to spend big in this month's Budget.

This all culminated with Health Minister David Clark on the weekend refusing to recommit to an election promise to bring GP fees down by an average of $10 by July 1. The change would still come - but it would be "phased in" over the three-year term.

Abigail Dougherty There is a lot of spin surrounding the upcoming budget. We try to cut through a bit of it here.

The reason for all this doom and gloom? Finance Minister Grant Robertson spelt it out in a pre-Budget speech on Tuesday morning: "We cannot make up for nine years of neglect in one Budget."

READ MORE: Robertson promises surplus

The Government's argument is basically that while the financial books might look to be in okay shape, there are actually billions of dollars of deficit in crucial infrastructure "neglected" by the National government - in health, in transport, in housing and in education.

National leader Simon Bridges rejects this, saying any issues in places like Middlemore are in the tens of millions or hundreds of millions of dollars range - while the Government has given itself an extra $10b to play around with by paying down debt slower. He says they are cancelling promises and raising new taxes because they've spent too much money on students with their fees-free policy. This is quite a different tune than the pre-election "$11b hole".

Meanwhile, commentators all over the place have criticised the Government's self-imposed Budget Responsibility Rules as a "fiscal straitjacket" - their argument being basically that debt is already low, there's plenty of areas that need money, and we should spend it now while interest rates are friendly.

So what's the truth? Does the Government have all the fiscal room in the world or have they spent it already? Let's take a look.

THE BIG PICTURE

Before we get down to the nitty gritty it pays to zoom out and think about the Budget as a whole.

The Government is expected to bring in about $77.5b in tax revenue for the year ending July 1, and spend $76.3b.

Much of this spending is broadly non-controversial stuff that would be happening, with some tweaks, no matter who was in charge - think Superannuation, which costs $13.7b for that year.

Generally in each Budget not a huge amount of those big spends are changed. But governments do have significant levers to pull with their operational and capital allowances each year, typically in the low single-figure billions.

SO HOW MUCH MONEY DID THE GOVERNMENT INHERIT?

Let's talk about this in four-year chunks, as governments generally do.

Labour came into power with Treasury expecting to earn about $335.2b from tax revenues from 2018-2021 and - if National's 2017 Budget stayed in place - spend about $340.3b.

In December Treasury changed these numbers to take into account some immediate changes Labour made when coming into power: their 100-day plan policies and the cancellation of tax cuts.

The Government was then expected to bring in roughly $341.8b and spend roughly $349.8b.

These "baseline figures" includes Labour's decision to pay off debt slightly slower and the spending on the 100-day plan policies, including $2b for KiwiBuild, $2.8b for the tertiary education policy, and roughly $5.6b for the families package of Working For Families boosts, the Winter Energy Payment, and other welfare boosts.

Much of the money for this came from cancelling National's planned tax cuts, which gave the Government $7.9b.

HOW MUCH IS LEFT OVER AFTER THE 100-DAY PLAN POLICIES?

To give governments room to change things and make announcements over the four-year period they allocate, as part of that $349.8b, both operating and capital allowances. This is basically new money that can be added to the ongoing spending (operating) and new money that can be spent on big new things (capital).

The Government gave itself an operating allowance of $2.6b for 2018, then $1.9b each for the next three years. Every year you have to add in the amount from the previous years, again, as the figure is cumulative - so the total is $21.6b over the four years.

So we can immediately expect a decent chunk of this Budget's operating allowance boost to go to the $1b Provincial Growth Fund. But once it is allocated in there it won't come out of the operating allowance for the following year.

Then we have the capital allowance - the Government has given itself $3.4b for the 2018 and 2019 Budgets, then $3.1b for 2020 and $2.7b for 2021, for a total of $12.6b.

This is the money that can be spent on things like new hospitals and schools.

For comparison's sake, National's projected operating allowances were $1.5b every year through 2020, and their projected capital allowance was $3b for the 2018 Budget and $2b for the years after that. In other words: Labour has allocated significantly more for both operating and capital expenses in each Budget than National did. Robertson also said on Tuesday the Government had found an extra $1.4b of savings and new revenue from small tax changes.

BUT ARE THE ALLOWANCES ALREADY USED UP?

Some of the allowances undoubtedly disappeared in coalition talks. You can assume a decent chunk of the operating allowance will go to the $1b Provincial Growth Fund promised to NZ First - but not all of it, as some of it will be capital spend. There's also the 1800 new cops promised which will be a pricey chunk of operational spend. And the Greens will be clamouring for more money for DoC.

But that doesn't mean there will be nothing left for health and education. Even as it downplayed the idea of cheap GP visits, the Government indicated its pre-election promise to increase health and education spending remained pretty intact. Indeed, Treasury estimated in December that even taking into account all of the coalition policies and Labour's pre-election fiscal plan there would still be $6.6b of operation allowances left off over the four year period and $8.9b of capital allowances.

That said - Labour's pre-election fiscal plan is no longer its blueprint, as it had to trim it post-coalition talks.

But it does give us a good idea of what the Labour Party wants to do: $8b more for health over the four years and $6b for education. These policies could comfortably fit into the roughly $34.2b of new capital and operational spend the Government has allowed itself.

(If you're wondering about transport, that's funded through a whole different system that comes from petrol taxes.)

So yes - there is money left in those allowances.

IF THEY HAVE ALL THIS MONEY, WHY IS GOVERNMENT DOWNPLAYING EXPECTATIONS?

While the operational allowances are enough to do plenty with, they won't be enough to do everything. Housing Minister Phil Twyford is very keen to double the amount of state houses being built every year, from 1000 to 2000, but it's not yet clear whether or not he's managed that. Pay rises for nurses and teachers can immediately eat up huge chunks of that operational allowance. But it is quite clear the Government doesn't quite have a bare cupboard. So why is it acting like it does?

Simple politics are a large part of this. After weeks of under-promising, Robertson may be looking to over-deliver. Bridges has indicated he thinks this is part of Labour's plan.

But it's also true that no Budget is going to give Labour, Green, and NZ First voters all the things they want. And it's true, as National have repeatedly pointed out, the Government spent some serious money the moment they came to power - on the fees-free policy, yes, but also on huge boosts for Working for Families, the Winter Energy Payment, and accommodation supplements. Labour made a big bang on coming into office, but it does give quite a bit less room going forwards than a new government might have liked.

Then there's this whole "fixing deficits" problem. Robertson has said repeatedly that a lot of money needs to go into simply fixing problems National left them. It's impossible, without actually being in Government, to really know if this is accurate - although it's true that the per-capita health spend levelled off during National's time in government. This could mean "horrific deficit" or it could mean "more efficient" - depends who you ask, really.

WHAT ABOUT THE BUDGET RESPONSIBILITY RULES?

All governments borrow money. They get some of the best interest rates in the world, can create their own money, have very reliable income, and often need to make investments fast.

While this Government is keen to pay off the debt a bit slower in the short-term, their Budget responsibility rules mean they have basically the same debt policy as National - if anything they are more fiscally conservative. The rules commit the Government to getting the debt-to-GDP ratio to under 20 per cent by 2022 and keeping core Crown spending at under 30 per cent of GDP. On current projections, the Government will do both of these.

The Government has come under fire from economists like Shamubeel Eaqub and Cameron Bagrie for these rules, which they liken to a "fiscal straitjacket". The basic argument is that since governments can borrow money so cheaply and since there are seemingly so many things that need money to work better - housing, health, education, you name it - why not throw off the arbitrary rules and borrow a few more billion?

This argument makes absolute economic sense. Even credit agency S&P - hardly a left-wing organisation - has said the Government could add a few percentage points of debt on without compromising its gold-plated credit rating.

But in political terms, Robertson knows that reneging on the debt promise without a major economic or natural disaster behind the decision would kill the Government's chance to hold a vast swath of centre voters. Labour already has to battle a perception it is more reckless with the economy than National. This would solidify that image for a generation to come.

No matter how big and complex the numbers get, this Government is still subservient to politics.