Digging up and selling our coal has a range of horrible environmental effects both short and long term. Short term includes: massive use of water that we need for agriculture and environmental flows, destruction of marine environment from dredging so coal ships can get in, giant holes in landscape. Long term: more of the same ocean warming that's killed about 50% of the Great Barrier Reef already. Google Adani coal mine for more on what Australians are pissed about. If you'd like a more strictly economic argument, the sale price sure doesn't cover the damage.
The thing is the railway line which is baseline to enable the development of the Galilee Basin was never really about Adani. There are other miners to gain from it who have poured a lot of money into the election of certain members of Parliament.
So, fix the taxes to correctly represent environment costs. The problem is not in the decision to dig or not to dig, it's in the costs of these decisions - and I think it's obvious that these two things have to be responsibilities of different government or private bodies.