I've bought a few cars from Victoria and driven them back here to NSW.
Generally I get the seller to cancel the registration (gives me grounds to negotiate the price lower as they get a refund).
Then apply for a permit. These let you drive around without plates for a week.
Generally with the RTA here, all you need is some kind of receipt to prove that you have purchased the car. There is no need for Rego papers.

I'd assume that it would be somewhat similar the other way around (Victoria --> NSW)?