As WA's finances rapidly deteriorated, the previous state government offered a range of explanations as to how and why it had happened.

Former premier Colin Barnett and his senior ministers argued their high levels of spending had been crucial, with a population boom and a surge in demand for essential services meaning costs rose substantially.

They said the collapse in GST and mining royalty revenue meant there was no way the state's debt and deficit problems could have been avoided.

But plenty remained sceptical about the claims and John Langoulant's probe of government finances cast heightened doubt.

So, with the benefit of hindsight and the findings of an official inquiry, how do the explanations the public was offered during those boomtime years stack up now?

Explanation 1: GST, iron ore price blamed

"The combination of 30 cents in the dollar for GST and a 60 per cent fall in iron prices has put us in this unavoidable and unpleasant situation." — Colin Barnett, May 2015.

Collapsing iron ore revenues were blamed for the state's budget woes. ( ABC News )

The then-premier and those around him attempted to paint the slide into heavy budget deficits and growing debt as an inevitable consequence of the sharp revenue write-downs that came about when the state's GST share and mining royalty income plunged around the same time.

The revenue fall was far-from-insignificant — with a 5.2 per cent, or $1.5 billion, drop in the two years after 2013-14.

But Mr Langoulant made the point that "revenues were plentiful" and, even after the decline, were still 37 per cent higher than they had been when the Barnett government came to office.

"Revenue was not the problem," Mr Langoulant said when releasing the report to the media. "The problem was the government spent too much."

Explanation 2: Bush spending 'worth it'

"The Western Australian government was responding to the local pressure of the mining sector in terms of infrastructure — providing power and water to Karratha so we could build more houses to get the rents down, and providing the extra dollars for the teachers and nurses to keep the public sector going … That is why debt has pushed up above $30 billion, and that is why I support every single dollar of that debt." — Brendon Grylls, May 2015.

The Ord River irrigation expansion cost much more than expected. ( Leighton Contractors )

Mr Grylls' endorsement of such a rapid ballooning in the state's debt raised eyebrows at the time, but his argument — which particularly related to the dollars that had flown into the bush — was that all the spending had been critical and it had all been worth it.

But Mr Langoulant found plenty of examples of spending from the Royalties for Regions fund, which Mr Grylls oversaw for much of the Barnett government's life, that did not deliver the desired bang for the buck.

The centrepiece aquarium of a $36 million research and education facility in Exmouth remains unfilled, a $35 million Port Hedland stadium has major flooding problems and projects such as the Ord River irrigation expansion and the provision of underground power in the Pilbara ended up substantially over budget.

Those are just a few of the examples of significant government spending which Mr Langoulant found was highly questionable.

"There was a significant deficit in the rigour applied to project selection and poor targeting of funding," Mr Langoulant's report found.

Explanation 3: 'No choice but to borrow'

"Faced with the explosion in demand from the huge population increase, this government had no choice but to borrow to build the infrastructure to meet that demand. We make no apologies for meeting that challenge." — Mike Nahan, May 2016.

Former treasurer Mike Nahan argued spending growth was unavoidable. ( ABC News: Eliza Laschon )

The former government argued that much of the growth in spending under its watch was unavoidable, with a population surge seeing costs in essential areas like health and education grow sharply.

Mr Langoulant's inquiry did find that a large portion of that spending growth went into the budgets of essential areas like health and education, but that much of that cost went to paying for higher salaries.

Capital investment also continued at high levels even once revenue crashed, Mr Langoulant found.

"Although population growth during the period … may have warranted investment in the state's asset base, decision-makers needed to be more conservative and questioning in their assessment of he need for long-term asset investment," the inquiry report stated.

Explanation 4: Debt is 'affordable'

"Current net debt levels remain affordable, with net interest costs as a share of revenue (for the total non-financial public sector) reaching 2.3 per cent in 2012-13, well below the government's target limit of 4.5 per cent." — Troy Buswell, September 2013.

Then treasurer Troy Buswell believed the level of net debt was affordable. ( Warwick Stanley: AAP )

As the growth in WA's debt accelerated, the Barnett government was keen to downplay the problem — insisting in the early years that the borrowing total was manageable.

One justification given was that with interest rates at relatively low levels, the borrowing cost was much lower than it otherwise might have been.

While the interest rates are still quite low, the cost of that debt has nevertheless grown to a substantial level.

This financial year WA is facing an interest bill of $895 million and annual interest costs are expected to grow to more than $1.3 billion by 2020-21.

For some perspective, the interest WA will pay on its debt in 2020-21 would be enough to build the proposed Thornlie-to-Cockburn train line and Byford and Yanchep extensions, with more than $100 million left over — at least according to the most recent cost estimates.