Through a combination of information I found here and elsewhere I was able to get 15% off WB Movie World (Gold Coast) tickets. This made the cost of an adult ticket just $70.54, by far the best value I could find for an 'on the day' decision.
Basically you can get the Infinite Rewards program discount without being a member, through Theme Park Tickets Direct.
Spotted that /velocity was the entry point for 10% discounts through Virgin's frequent flyer program, tried my luck with /infiniterewards and it worked.
Direct link for 15% off is therefore: http://www.themeparkticketsdirect.com.au/infiniterewards
I purchased 2 tickets through the link this morning and got into the theme park no trouble a few hours later. No questions about my Infinite Rewards membership - I don't have one. Just had to exchange the printouts for tickets at the normal sales booths.
May be worth experimenting with some more URL entry points. It's a poorly designed system and who knows, there might be a 20% off program out there.
I dont fully understand the pricings of these things, but you might want to check something that was in the Sunday Telegraph.
It says an annual pass to Sealife, Wild life, Madame Tousards, Underwater world, Melbourne aquarium, Sydney tower & Illawarra Fly, plus 2 NZ attractions
Can be found for 20% off at www.merlinannualpass.com.au Viz $80 adults $56 Kids or $236 Family of 4
Might suit those who aren't into going to the Gold Coast