I wrote off my first car........ a ford laser no less than it deserved :LOL: Anyway the plates have to be handed back into the Transport Department in your state. You will also get a refund on your rego if you have a few months till expiry. I suppose there is nothing to stop you transferring them to your new car though.
Don't know if it's the same everywhere... but here in Tassie the bad news is that they have to be handed in to the Motor Registry ASAP. The good news is that they pay you back the unused reg on a pro-rata basis (or at least that's what they did 10 years ago then I had a car written off). I think that personal plates can be held by the registry in your name but unassigned to a vehicle until you have a new vehicle to attach them to. I believe its an offence to keep the plates yourself as they could be used fraudulently.