Scathing review into billion-dollar GST deal that which sees WA get 75 cents in every dollar, has been dubbed a "costly mistake".

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.