Well, the first thing you need to bear in mind is that there are numerous reasons why a company might have a job ad out there. Maybe someone has just resigned and they need to replace them. Maybe they need to bring in some new skills that they don't have in their current team. Maybe business is growing and they want to increase the size of the team. All the different reasons will give different levels of urgency.
So, there's no one general answer to the question of how long is an employer willing to wait.
In general, I would say employers in Australia would like you to be available to start within four weeks of accepting an offer, since that's a pretty common notice period in the software industry. But if there's no specific urgency (e.g. they're just looking to increase the size of a team), and they think you're the right candidate, anything is possible.
Two and a half months is a long time. But given that anything is possible, I would recommend that if you see a job ad which seems particularly tasty, get in touch. But make it clear in your cover letter that you are not available until October. If that's a deal-breaker, then it's a deal-breaker. You won't get the job, but no harm done. Better to make it clear immediately and not waste your time and theirs on an interview.
It's very possible that it's not a complete deal-breaker, but that it will hurt your chances and make them prefer a similar quality candidate over you. There's not much that can be done about that. I really wouldn't recommend keeping quiet and hoping to get an offer and then breaking the bad news about your availability. That's going to leave a bad taste in any employers' mouth.
Plus, of course, it's possible that your availability will get you ruled out.. but when August rolls around, then September, and they still haven't found a good candidate for the job, they'll get back in touch with you!