You will most likely have to drive it on the present rego as most states won't let you transfer the rego to your name unless you have a licence and address in that state or territory, and of course then the seller is taking a risk on getting fines in the mail for your tolls/red lights/speeding etc
Happened to me in Qld without the car I sold even leaving the state I got a toll bill because transport were slack to transfer info to toll roads.
And you will then most likely have to get a new safety certificate/rwc to reregister it in WA ??
Or you could get permits but they would have to be for all the states you are travelling in.