Why is mining such an important sector in the Australian economy?

Economics isn't my long suit, but as I understand it, only mining profits are counted in the GDP figures and that's only a small part of the equation. Mining contributes mive royalty revenues to several of our states and the industry is a big spender on rail freight, shipping and other goods and services as well as being a very significant employer. Also, over 30% of our exports come from the mining sector which is critically important to our terms of trade given that we are a nation of high imports.