Another alternative (and I have done this MANY times myself,. and it is well worth it) is to get to the Hume Freeway, go to Goulburn, then head up through Oberon, eventually meeting the Great Western Highway near Hartley. This will bring you back through the Blue Mountains to Sydney. via Katoomba.
you'd probably want to schedule a night in the upper Blue Mountains though as it will be a bit of a drive from Goulburn to Hartly via Oberon. although you might be able to schedule a Jenolan Caves visit in with that.
Dave