One of the government's targets for mining is the Waitutu Forest in Fiordland National Park. National wants to drill for oil there. The area is in a national park and a World Heritage Area, which ought to be a clear no-no, but National's spin is that they will only be taking a little of it and that it therefore doesn't matter. The problem is that it does. The whole area is of extremely high conservation value: it is the largest area of continuous lowland forest in New Zealand. While some parts of it were logged early last century, most of it is untouched, and more importantly, unfragmented by roads. Its value as an undisturbed habitat is immense.
Drilling, or even exploration, will destroy that value. Quite apart from the direct impacts (forest clearance, oil spills etc), any drilling or exploration will require heavy equipment to be moved, which means they will need to clear roads, which will destroy forest and (more importantly) fragment the ecosystem. And while the mining companies will say they can put it all back again when they're done, that is simply a lie. Once they've dug it up, the area will take centuries to recover.
That is what is on the line here: some of our highest-value conservation land. National wants to despoil it for the profit of the few. Instead, we should keep it intact to be enjoyed by the nation.