They can put it back in the system.
I bought a car for parts (with rego on hold) and when it arrived I decided it was too good so a couple of weeks later I go to get it a wof and told its been dereg, wtf I thought, called up ltsa about it and the previous owner dereg it after I bought it so ltsa reinstated it with a few clicks while I was on the phone to them. If only it was that easy to rereg a car thats just fallen out of the system