I was going to buy a skyline from NSW so i investigated this a while back. I was going to fly up, buy the car, get proof of purchase, drive down to vic.
Then hand the NSW plates in to Vicroads, they give the registed owner the refund (the old owner), then you have to get a Vic RWC and get new rego. So its still registered to the old owner when you drive it interstate, but as long as you have proof of purchase you can hand the plates in yourself.