Hey (westcott),
I'm a bit of Qantas snob so looks like i'll be paying $600 +.
Have you checked Tiger? You could do a Virgin,Tiger (or Qantas) mix. Sometimes that can work out really well.
Tiger leaves from PER at 7pm for MEL for $248 (inc taxes)...maybe not then? The MEL to PER leaves Melbourne in the evening, so no go there.