I know this was posted a few years back, but I found it helpful before I went to do the theme parks last week and thought someone else might need to read it one day. I am 166cm tall, weigh 105kg (but lost 4kg last week just from running around like a lunatic when I was up there!) and wear size 16-18 pants and size 14 shirts.
I didn't get questioned by any of the staff and I was able to go on all rides without a fuss. The Claw and the Motorcoaster at Dreamworld were the only rides I felt slightly uncomfortable on (motorcoaster only because I have big calves, but still I got on without a fuss), but apart from that, all was fine. Giant Drop's seats are exactly the same as The Claw, so if you can get on one, you can get on the other. In saying that I felt slightly uncomfortable, there were much, much bigger people than me getting on and were perfectly fine. Cyclone is very roomy, as is the Wipeout, and there was still PLENTY of room on the Buzzsaw.
I hold most of my weight in my bum and thighs, and was very worried about the Scooby Doo and Superman rides at Movieworld after reading reviews, but I didn't have a problem with either of them. All other rides were fine, and I didn't have an issue fitting on any of the other rides.
Rides at Wet'N'Wild, Seaworld and Whitewater world were not an issue. There are weight restrictions on the slides at Wet'N'Wild and Whitewater world, I think 180kg combined weight, and you have to be between 45kg and 120kg to do the aqualoop at WNW (you have to stand on a scale at the beginning of the lineup for this ride). Apart from that, as long as you are comfortable and the restraints fit properly, you will be fine.
