Best advice as always is to shop around. Check as many places as you can.
Dr. M suggested this site a while ago on AAM for US flights that is very handy.
It has definatly gotten me the cheapest flights on a couple of occasions (I've always done a comparison with other search sites and company home pages), but never tried it for Oz.
I've booked return flights from Aus to Ireland in the past with Zuji (an online Australian travel agent) as they were the cheapest I could find. Don't know if they'll sell you a flight going TO Australia but I don't see why not.