The High Court is expected to deliver its verdict on the legality of the government's controversial Mineral Resources Rent Tax (MRRT) on Wednesday.
That follows a challenge headed by miner Fortescue Metals Group, even though the company hasn't paid any of the tax and doesn't expect to for a number of years.
The MRRT, which came into effect in July 2012, imposes a 22.5 per cent tax on profits over $75 million derived from the extraction of iron ore, coal and coal-seam gas.
Miners could offset MRRT liability against expenses including state royalty payments.
Mining companies, along with the states and the opposition, vehemently opposed the tax.
In the High Court, Fortescue argued that the MRRT contravened constitutional requirement that taxes be levied uniformly in that it differed in each state because of different state royalty rates.
Should the MRRT survive this legal challenge, it faces further peril with opposition leader Tony Abbott vowing it will go if he wins government.
As it turned out, the economic downturn in the resources sector meant the MRRT produced far less revenue than anticipated - just $200 million in its first year and $700 million in the second year, according to budget figures.
Last October, it was forecast to raise $2 billion in 2012-13 and $2.4 billion in 2013-14, and those estimates were actually a downgrade on the previous budget.
