I hate to say it, but here in the US, just because a horse is bred to be bronc horse for rodeos doesn't mean the horse is dangerous. I know several bronc horses who after their careers were sold and turned into great riding horses. Once was a top notch barrel racer, a couple were fun trail horses, and I know one who's owner started jumping him. No idea how he's doing now though. I myself looked at a few ex-broncs. It sucks that it seems like this person lied to you, and after reading your other post, it sounds like she wanted to make a quick sale, and you were so interested that she decided to go for it so she could get a few bucks, maybe pay off a bill or something. That doesn't make what she did right, but there were quite a few red flags, and you chose to ignore them. Your posts and comments seem to be a little contradictory, first she was starting to do better, then it sounds like someone recognized her, and told you she was "dangerous", so you decided to agree, especially after she acted up after being off for a bit. My first thought would honestly be to get a chiropractor and saddle fitter out, and make sure that the equipment fits properly, and the horse isn't acting up out of pain, then start taking other steps.
If the seller can reasonably sway the court to believe that she told you to get a saddle fitter out or something (whether she did or not is a moot point), that she cautioned you at all about the horse, and possibly needing work etc, chances are you guys aren't going to get a dime in repayment for false advertisement. Hopefully something can be figured out, but here in the US, it pretty much is buyer beware, and if you bought a horse that was too much for you to handle, no matter what the advertisement was, as long as you weren't injured, you aren't going to see anything, its on you for buying the horse. No idea how it is in Australia, but I'd be prepared. Especially because if she lied about what the horse can do, then she'll probably lie in court too, so no guarantees that things will go your way. Sorry that you've had to deal with this, but maybe next time you look at a horse you'll get a more professional opinion before buying.