Mmm... lets have a look...
The site might be potentially liable as the Act has an extraterritorial provision at s8(2)(b). That means Victoria may prosecute for acts committed outside Victoria if the effect of the act is felt in Victoria and they can get a hold of the culprit. So, probably, a complaint is possible if a breach of the Act can be demonstrated - but the Act is for really serious hate stuff (think Nazi), not people blowing off steam amongst peers and like minded people as happens here.
Also, the site is well moderated so the owners probably have protection under s27(4) as they've taken steps to prevent that stuff, and also there are exceptions at s11 to provde for democratic free speech including legitimate discussion. Also, enforcment would be hard as I doubt the UK would extridite Simon becuase Farkel or someone posted some cutting remark on JWD!!!!
So, no... Simon and posters are probably safe but there is potential for problems if they read the plans here for bombing the Assemb... nah, they might take that the wrong way :-)... so long as it dosn't become a real hate site and noone's hatching plans for really bad stuff, then there shouldn't be any issue at all.
Cheers, Max - who's just exercising his mind and is probably wrong about everything above!!!!