I echo willyroo's comments. The Melbourne Westin is very nice and a little more centrally located than the Sheraton, although the Sheraton is closer to the Casino and Southbank which has some great eating opportunities. When in MEL, make sure you eat at Lygon St at least one night. Hotel concierge will offer specific suggestions. It is a short cab ride and you won't get ripped off by the cabbies.

Around MEL, also try to fit in a trip through the Dandenongs (mountains). There are lots of options, including the Puffing Billy train. The Fairy Penguins at Phillip Island are also worth seeing, but dress warmly as it can get very cold at dusk down there.

The Brisbane Conrad hotel is part of the casino complex. If you like the Hilton chain (as reflected by your choices mentioned) you might also like to try the Brisbane Hilton which is located right on the Queen St Mall. There is a current post in the Hilton forum about it. But by all accounts the Conrad is very good.

There are less touristy things to do in BNE than in and around Sydney. In BNE, I recommend taking the CityCat (ferry) along the river and have a walk through the Southbank garden area (either walk over the new foot bridge or use the CityCat). Mt Cootha is work a look if you can get there (rental car, tour, public transport). Otherwise most of best things to see are outside the city. If you have a rental car, then Montville is wonderful. As is Australia Zoo (Steve Irwin's zoo about 45 mins north of Brisbane). Then of course there is the Gold Coast and Sunshine Coast.

From Sydney, the Blue Mountains are a must. I am sure thadocta can recommend a means transport through that region - that is, unless all his cabs are busy delivering beer . He will be able to offer plenty of tips.