transfer it, then 28days for RWC before reg is suspended, i assume you lost the RWC, as it is illegal for a car to be sold without one.
editing for others reference: if car is unregistered, the only time limit you have is the expiration of unreg vehicle permits, and RWC's. you dont need to transfer an unreg car as such, just reg it in your name, providing a receipt for the purchase. You make an appointment at vicroads, take in receipt, RWC, anything else they need then fingers crossed you drive away with plates on.