I'm just a yr 12er (HSC almost over yay!..except eco) ...so keep that in mind but from my little bit of research about each of the unis here's what I've noticed.
ANU: Thier Eco is unique in the way they offer an optional "honours stream" for most of thier eco courses, special additional classes that are designed to challenge students interested and want to learn more of a particular area. Similar to how B Advanced sci is different from normal B Sci
They also allow students to focus on a particular of eco apart from thier eco majors, eg Business Eco, Eco Development, Eco Theory, Eco History, Money and Banking
USYD: The Good Uni Guide 2006 has them having a highest employment and graduating salaries of all unis ($50K as opposed to $40 something thousand for most unis). Apparently they are good, as you would expect
UNSW: I don't know much about them but they let also let you specilaise in a particular area of Eco, (Eco, Econmetrics, Eco History, Financial Eco, and Business Strategy and Eco - the last is only avaliable through B Com though)
USYD and UNSW are more flexbile, especially if you're intending to do a double degree with Eco IMO; ANU forces you to do a Eco major then specialise through additonal courses, while UNSW lets you specialise in a particular area of Eco right away.