We wanted to rent a campervan in Australia (Brisbane-Cairns, 10 days), but they want 7500 for a bond!!! So we decided we'd rather travel in a standard car and sleep in hostels, but they still want 2500 as a 'deposit' or you need to pay insurance, which is another 250! we hired a car in Greece and they did not make us pay such a ridiculous sum of money (and no-one guarantees they will be honest when paying this back. any advice?
Most places (at least the big companies) should take a creditcard imprint as bond instead, and nothing should be charged to that unless there's actual damage. I don't know who you're trying to rent with, but try carhire.com.au to find an alternative, maybe. (That site only searches the big names, but gets you pretty good rates all the same, in my experience.)
Shop around. You're spoilt for choice in Oz: Spaceships, Jucy, Travellers Auto Barn, Wicked, Hippy Campers, Backpacker Camper Vans, Britz, Apollo, and so on.