Hi,
I did it, but it wasnt a campervan, it was an estate car with all the camping equipment (and was kn@ckered, but thats another story ....) If you go to hostels, which no doubt you will, you will see loads of adverts for them on the wall. Usually they are other, like minded travellers, who want to sell their van/car after travelling for a while.
In Melbourne, there are loads of cars left on the streets sometimes, as backpackers have gone home and abandoned them. If Im being honest, and i dont recommend this, I never registered my car.
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Im sure this is very naughty, but I never got asked any questions about it, and gave it away at the end of my trip.
Anymore questions feel free to ask