Apollo Motor Homes Australia has branches Adelaide, Alice Springs, Brisbane, Broome, Cairns, Darwin, Hobart, Melbourne, Perth and Sydney and frequently advertise relocations at their website. You are able to book relocations directly at the Apollo Motorhomes website, Normal cost for a relocation will be $1 per/day, sometimes extra days are available for $75 per/day (up to 3 days) depending on how soon the vehicle is needed at the destination. Sometimes relocations also include a fuel allowance, so make sure thats noted on your rental agreement and make sure you get a tax invoice when you fill up the vehicle. If they are in a real hurry for the vehicle you may also get $100 no receipts required. Other things to note you have standard liability of $1000 it can be reduced for $20 Per/day and you are not allowed to travel on unsealed roads on a relocation vehicle even if it is a 4wd vehicle.
http://www.apollocamper.com/reloc.aspx