Heh Trieu, that's our secret about the location.
Oh, alright, go to the steps of the OH, look back towards the park and you'll see some steps leading up to the park. Half way up the steps there is a big gum tree (also lit up at night, I have some shots of it somewhere), and you can climb onto the ledge near the gum tree. Hang onto fence with one hand and work along ledge for a couple meters (away from bridge) and the ledge widens to about a meter wide. Plenty wide enough to set up a tripod. We found a visiting photog from overseas there, so it's not too hard to find.
By the way Trieu, they are great shots. Sharp, nicely exposed (maybe a tiny smidgeon over), great colours. In my opinion, no need to swap the Sigma for the Tokina. The Sigma is working just great for you.
Cheers
John