The new Hilton SW is a bargain -- it will be priced like the Westin once it is better established. Location is ok. Otherwise MEL is awash with 4* hotels priced AUD 250-350, plenty to choose from even during the Open. However prices will go up. Grand Mercure on Swanston is a good choice. Also Australia has a lot of excellent serviced apartments (priced like hotels, but you get a kitchen). Consider the Medina or Mantra chains. Finally you do have some non-CBD choices that can be excellent, for example the Novotel on St. Kilda Beach has the 96 tram stop outside the door and is 15 min from CBD by trams that run every 8 minutes most of the day. Or stay in South Yarra, at the Como though that will cost more and isn't really better.