Hey there, glad to see you taking an interest in academia. I hope I can offer some valuable advice (the below will be for economics/business - finance, management etc).
I am currently doing a PhD in economics at MIT, I was also admitted to Stanford, Yale, Princeton, and others. Hopefully I can give you some insight.
First, the admissions process. Graduate admissions (for economics and Business disciplines such as finance etc) are VERY different from undergraduate and are borderline "impossible" to get in (for the top 10s), unless you are a super-star. Just to give you an idea, in my year, there were about 1000+ applicants for a class size of 10. Business schools are even more atrociously hard to get in at the top 10. In finance, MIT Sloan had about 1500+ applicants for 3 spots, that's a 0.2% chance of getting in. This is not to discourage you from trying, but just remember, do not only aim for the top 10, make sure you have "safety" schools, those ranked in the 20s, 30s, 40s etc. But, as I will discuss later on, prestige in academia matters, and when I say matter, I mean they matter alot, so doing a PhD at the top 10s such as Harvard, MIT, Stanford, Berkeley, Chicago, Yale, Princeton, etc make a huge difference.
Now, on to the actual admission process. First, is your academic transcript. Needless to say, you need to be stellar. Anything short of stellar and your chances are severely crippled. This means you need to be the University medalist, win unit prizes, scholarships, have close to a perfect GPA. Especially coming from Australia, you will be competing against the best of the best from across the globe (e.g., some are gold medalists in multiple IMOs). Having a stellar academic transcript is only the "baseline" requirement, it is a necessary condition but nowhere near a sufficient condition. Almost all successful applicants in the top 10s have amazing/out of this world academic transcripts. Second, the GRE/GMAT, you need an almost perfect GRE/GMAT to be even considered. Anything short of a close to perfect quantitative score on these tests and you will be thrown in the trash bin (unless you have spectacular connections, more on this later). Third, research experience. You need an abundance of research experience, try find RA positions. Try work with renowned professors in your field. Most importantly, try aim for publications in top journals (this is extremely, extremely, extremely hard), but if you can publish in decent journals (e.g., ABDC ranking A) before you apply, you will be a stand-out star. Certainly, not everyone has publications and it certainly isn't a necessary condition, but if you do, you will be noticed. I personally had 4 publications (two A* ranked, two A ranked) before applying. Fourth, your recommendation letters. This is perhaps the single most important part of the application. It is almost impossible to separate out applicants based on the previous points because everyone is so spectacular at the top end. Having incredible recommendations is crucial. So what makes an incredible recommendation letter? This is where connections come into play. Your recommendations letter write must be a renowned professor in the field, do not have an assistant professor or associate professor write your letter even if he can write a stellar letter. Connections are super important and unfortunately, at least in economics/Business, this is where Australia lacks behind. I am not saying the Professors here aren't "well-known" but when compared to the top 10, Australia academics are often "disregarded" as they unfortunately not recognized as being at the same level as the top institutions in the world. To "overcome" this hurdle, I highly suggest you search for RA positions in the US and work under top Professors for at least a semester so that you can build relationships with them and have them write your recommendation letters. This will be expensive, but consider it as a worthwhile investment, especially if you are aiming for top 10's. Fifth, your statement of purpose, to be brutally honest, this is the least important part of the application. Obviously write a good and proper statement of purpose, but if you are lacking in any of the previous criteria, your statement of purpose is unfortunately not going to be read.
In summary, to have a high chance of making past the admission process: 1) Close to perfect academic transcript (University medalist, prizes, scholarships, perfect GPA); 2) Close to perfect GRE/GMAT scores; 3) Publications; 4) Top tier recommendation letters by well-known Professors.
Second, what is doing a PhD at a top 10 actually like? Very different from Australia, and rightly so. The standard is light-years above that of Australia. Usually PhDs in USA are 5-6 years. First two years are coursework, after which you will have a qualifying exam. In econ/business, you will usually take the standard micro/macro sequence, as well as other sequences if you are interested; econometrics, labour etc. Will also take maths courses such as measure theory, topology, etc After the two years, you will be doing research. Again, the standards are drastically different from Australia (much much higher). Advisors are all top-tier world-renowned academics, top of their field, and the end product will usually be published in A** journals. Top 10s are just a league in its own, not a single faculty in Australia can compete with their research output. Funding at the top 10 is also insanely generous. Everyone is on a full scholarship with stipends ranging from 30-50k USD p.a.
Third, job market prospects. This is where the prestige factor comes in. If you wish to have a carer as an academic at the top 10-20. Unless your PhD is at a top 10, you will have almost a zero chance (barring exceptional circumstances, e.g., advisor is a Nobel Laureate etc) to make tenure at top 10-20s. Prestige matters a lot. The placement record at top 10s strongly dominates that of anywhere else in the world. This is a combination of research excellence, connections, politics, and inherent self-selection bias. If your aim is to work at a top 10-20, do not do your PhD in Australia or anywhere else. I would suggest that if you do not make it into a top 10 program, do not bother to do it at top 20-30s, unless academia is your dream job and you are willing to work at lower ranked institutions.
Fourth, assuming you do make it into the top 10 and do get tenure at a top 10-20, life as an academic is one of the best jobs, especially in econ/business. To give you an idea, most top 10 academics make around 500-600k usd p.a., lifestyle is great, you have plenty of time with family and friends, teach classes perhaps 2-3 times a week, and you can spend the rest of time anywhere you want (if you like to do research in the comfort of your own home, do so). Combine this with at least 3-4 months of vacation in a year, along with sabbaticals etc, a tenured professor at top 10-20 is one of the best jobs in the world when you consider pay vs time ratio. Obviously, the road to being one is extremely challenging and very few make it.
Finally, the advice above is for top 10s only, lower ranked institutions obviously have lower requirements, but at the same time, your job market prospects are lower, future career is more uncertain, and lower expected salary. Obviously, you could still turn out to be a star, it's just that your chances of doing so are much less.
Feel free to PM me if you have any more queries or just post in this thread.