No, you can purchase plates just for fun, or as a collector's item. Once you lodge your Remove and/or Attach form with the Department of Transport and Main Roads, you will have to attach your plates right away and you must take your Standard Issue plates in with you to be surrendered.
If you've upgraded, redesigned or replaced your current personalised plates, make sure you return your original plates to a Transport and Main Roads Customer Service Centre or Agency within 14 days of receiving your new ones. Your redesigned/replaced plates are able to be put straight onto your vehicle when they arrive!