As someone who's routinely paying extra for everything from coffee, to bus rides to hotel rooms simply on account of being a foreigner, I'm all for charging tourists double to access New Zealand's Great Walks.
These are our prime jewels and as such, we should value and appreciate them enough to demand good coin from anyone fortunate enough to visit them. Charging for something awesome — and it's a word not used lightly when discussing the Great Walks — makes more sense than banging dumb taxes on airport arrivals and hotel beds, with no discernable return on investment for the visitor.
But if they're paying more, the onus is on us as hosts to deliver more; to put the profit into maintaining the awesomeness.