The waiting time will be approx 4-6 weeks, as they will have to re-make them.
You will probably have to pay about $16 for a set of standard plates while you are waiting for your ones to be remade, as well as another $175.40 to have polycarbonate ones re-made.
Check out this site to deal with lost/stolen plates. You have to go fill in a statutory declaration.
http://www.dpi.wa.gov.au/licensing/customplates/1400.aspAfter seeing those proces, I'll be looking into keeddlez's idea of security screws.