The interim report, released on Friday, labelled the 2018 deal a “mistake” that could end up costing Australian taxpayers up to $12 billion a year and is failing its key objectives.

The 2018 deal by the Liberal party under Scott Morrison to shore up political support in the west is costing federal taxpayers more than $6 billion a year.

The GST system has become so broken that a bushfire in NSW delivers a financial windfall to the taxpayers of Western Australia.