Unless it's in the process of the vehicle ownership being transferred, then you need to wait for the logbook to be returned to you and have a current retention certificate. Once you have the new log book for the car just purchased then you can do it instantly. It's a longer process if you need to take a private reg off, wait for the log book to come back with the original registration and then assign a new one as it can't be done without the logbook. It's a lot quicker than the old way of going to the local office though.