Are questions posted on aviation.stackexchange.com designed to be applicable to the U.S. only ... or are questions for other locales OK? If it's worldwide, perhaps there need to be some country tags.
What's happened here is that you're the first person to ask a question about Australia!
We don't just create tags - we only create one when a question requires it. And until you have enough rep, you can't create tags (you only need +8 more — only 150 is required while we're in beta).
As you rightfully asked in your question, "can somebody with rep add this tag for me?", I've created it for you now.
After further research, the consensus seems to be that questions are not limited to the USA, however posters of questions should indicate the country (or regulatory body) that their questions apply to.