I haven't, but have friends who have gone twice. (We were headed that way before we changed ALL our wedding plans.) From my friends: the trip there and back is tedious, to say the least, but a trip including Australia and New Zealand sounds AMAZING. My friends did both on both trips, but stressed DO NOT SKIP New Zealand! I have 2 wonderful trips coming up, but I am still just a little jealous of you, lol.
I went there and New Zealand! I LOVED it!!!! We were in Cairns (Great Barrier Reef), Sydney and Melbourne then in New Zealand we went to Christchurch and Queenstown. I loved Cairns but that's because I love the beach and laid back areas. We went to Green Island to snorkel and it was gorgeous. Next we went to Melbourne and I really liked it there! Lots of shopping, great casino, wonderful city to walk around in. While we were there we also took a day trip over to the Twelve Apostles. AMAZING. I'd have to say snorkeling the Great Barrier Reef and taking a helicopter over the Twelve Apostles were the best parts of the trip. Sydney was alright. You see the Opera House and the bridge and then that's about it. We went to 2 different beaches there (taking ferries to get there) and that was fun! Christchurch was pretty but boring. Queenstown was nice but I feel like it would be more of a winter destination. I highly suggest going! It was incredible!!
I have been to Melbourne a couple times and Sydney. I agree with Lala Sydney is over rated. But I love Melbourne. We are going there for a week. We are planing to spend part of our time at Philip Island - its just off the Melbourne coast - just a small bridge away. You can see penguins there - and they are awesome. The the 12 apostles are fantastic. I recommend the tour of the great ocean road - with the helicopter like Lala said. Also make sure you hit Max brenners for a chocolate crape better there then in NYC.
Sorry it took me so long to get back to this!! St. Kilda is a beach area outside of Melbourne. We really liked it there. Fabulous restaurants, laid back vibe. Manly is outside of Sydney and it was nice as well. I prefered St. Kilda. Another a little further outside of Melbourne is Lorne. We didn't actually stay there but just had a very quick stopover. Very artsy and quaint. Good luck!!