They were quite a sight, all those coalition MPs lined up in Perth on Thursday popping the metaphorical champagne bottles over the GST shake-up.

They all wore smiles like split watermelons.

And so they should have.

The extra $4.7 billion flooding into the State pot over the next eight years should buy a hell of a lot of votes.

But there was one MP in the gaggle who should have felt much happier than the rest — Attorney-General Christian Porter.

For him, this is a victory on two levels. First, it gives him a better chance of defending his seat of Pearce. Neutralising the atomic issue of GST inequity means there is one less thing that could blast him out of his electorate as he stares down a changing demographic that naturally benefits his Labor opponent.

But there is another reason for Porter to feel the comfort of a reassuring wave of relief.

It was he as WA treasurer who made one particularly ill-advised decision that magnified the State’s fiscal malaise — a decision that Federal Treasurer Scott Morrison’s GST reboot now effectively corrects.

It was an act that implanted a massive miscalculation at the heart of the 2011-12 WA Budget, which has plagued each subsequent State economic statement.

Oddly, it began with a prudent prediction on the precipitous drop in the State’s GST allocation after the mining boom. Porter was overly conservative in forecasting that the Commonwealth Grants Commission would reduce the State’s relativity to as low as 33¢ in the dollar by 2014-15. The actual outcome was 38¢.

His next prediction, though, was far from prudent and close to reckless.

In his 2011-12 Budget speech, he said: “What we reasonably anticipate is that in 2013-14 the Commonwealth Grants Commission will have brought in a new GST system. We expect it will produce a floor of about 75 per cent of our population share of the GST. Therefore, we expect extra revenues of $1.8 billion in 2013-14 and $2.5 billion in 2014-15.”

That reasonable anticipation turned out to be utterly unreasonable. The grants commission didn’t bring in a new system and there would be no floor. Indeed, Federal Parliament, including all Porter’s coalition contemporaries, voted against one.

But his decision baked in a fatally flawed assumption that allowed the State to spend billions of dollars it was never going to see, inevitably racking up more crippling debt.

In its draft report into the GST, the Productivity Commission reflected on Porter’s actions by noting with customary bureaucratic restraint: “WA’s experience has been an unprecedented outlier, exacerbated by earlier Budget decisions of the WA government.”

It was referring, however obliquely, to one decision in particular — Porter’s.

The Productivity Commission then allowed respected economist Saul Eslake to be more direct, quoting from his submission: “WA’s present fiscal woes are the result not of a flawed system of distributing revenue from the GST among the States and Territories, but rather of its inability to control its own spending.”

There is some mitigation for the WA government’s spending in those years, but none for Porter’s blunder.

GST payments are calculated over a three-year period with a two-year lag in payments. The grants commission insists that doing it this way allows it to make a more precise assessment based on a comprehensive set of empirical data.

So, during the minerals boom, WA was reaping the benefits of rich iron ore royalties and handsome GST payments at the same time. The Productivity Commission found the State raked in $7 billion more in GST payments in that period than it would have if the calculations had been made on a same-year basis.

But WA probably lost about the same amount coming down the other side as its royalties and GST payments collapsed simultaneously and put the State finances in a death spiral.

Eslake notes that while the WA government’s total revenue per head of population during this period was just 0.7 per cent lower than the average of all the States and Territories, its per capita spending was 10.5 per cent higher.

In isolation, that would suggest massive fiscal negligence by a government spending money in an arterial gush knowing that its revenues were going the other way.

Yet it ignores the complex realities facing the government at the time as it struggled to keep pace with an unprecedented boom that was driving prices, wages and, hence, the cost of service delivery through the roof.

There is absolutely no doubt, though, that Porter’s massive miscalculation put the State further into the hole than it otherwise would have been.

About $4.3 billion further, to be precise, which is just a little bit less than the additional funding the State will receive over the next eight years as the grants commission transitions to the new system.

This time a relativities floor will be more than a reasonable anticipation, it will be a reality.

It will start at 70¢ in 2021-22, the year WA’s relativity is predicted to recover to that level anyway. It will then rise to 75¢ in 2024-25 as the Federal Government’s annual top-up payments climb towards $1 billion.

Scott Morrison was right to reject the Productivity Commission’s recommendation for the new payment relativities to be calculated against a baseline of the average of all States and Territories.

WA would have eventually done better under that system, but six other jurisdictions would have been left worse off — Queensland and South Australia considerably so.

Morrison’s alternative, using the performances of NSW and Victoria as the baseline, is considerably more reliable and equitable. WA is still by far the biggest winner.

It is an elegant and sensible solution. Despite the mutterings of some of the McGowan Government’s Labor State contemporaries, the odds strongly suggest it will get unanimous agreement.

As Paul Keating once observed, you never want to stand between a bunch of premiers and a bucket of money. The present bunch will eventually dive head-first into this bucket.

And one of the most relieved people in Australia when they do will be former WA treasurer Christian Porter.