Not open anymore
By A Yahoo! Contributor, 11/25/10
Me and my friend went to the Rialto towers for the observation deck, but apparantly they've closed it - they referred us to the Eureka Towers just across the river - so we went up to the Skydeck on Eureka tower and it was amazing, it was just around 17$ for adults and 12$ for the Edge, but that was optional (a box that goes out 5 mtrs from the building).
A bit prizy but still a good thing to do - since it is the tallest skydeck in the southern hemisphere.