Found this via travelzoo, but checked on the attractions' websites direct and it's true. Basically if you go late in the day you'll save 50% on the tickets. Personally I'd prefer the Tower after 7pm, on a clear night you get a beautiful view. Aquarium only ever takes me about an hour to go through, so no probs to go after 4pm.
The following attractions are available at discounted prices:
The Sydney Tower Eye (after 7pm) … $12.50
Wild Life Sydney (after 3pm) … $17.50
Sydney Aquarium (after 4pm) … $17.50
Offering 360 degree views of Sydney, The Sydney Tower Eye in the city centre is the ideal place to get a bird's eye view of this vibrant harbour city.
Wild Life Sydney in Darling Harbour allows visitors to see Australia's iconic animals up close, while the nearby Sydney Aquarium is home to over 12,000 aquatic animals and features an underwater viewing tunnel.
This offer is valid for visits on select dates until 31 October, 2012. Available until sold out.
I might buy Sydney Tower one