As a (cheaper) alternative, you might also want to consider going up the Eureka Tower (tallest building in Melbourne, around 300m), which has amazing views over the city on a clear day (which is most days in Melbourne).
http://www.eurekaskydeck.com.au which takes you out into space, almost 1000 ft up. It's a great experience, but you need to be OK with heights.
A must while you're in Sydney is the bridge climb, which doesn't need that good a head for heights, just a reasonable level of confidence say with a tall ladder. Given that they are obsessive with safety, it's totally impossible to fall anyway. The only drawback, is that it's quite expensive at around 220 Aus Dollars (more at twilight).
Enjoy Australia
Stuart