So, as a collective, our answer to this question tends to be 'don't do academic history as a career'. While most of the best advice previously handed out is from an American perspective, the same broadly applies to Australia (although at least the financial side of postgraduate study is generally less punitive). You are taking a big gamble by making the choice, and it needs to be done with the knowledge that your success will come down to luck. It doesn't matter how good you are, how hard you work, how well you plan - it can still go wrong.

Specifically with regards to the University of Sydney, it's a good uni, has good historians working there and (*cough*) good historians who have been trained there. It will take effort on your part to stand out both academically and individually - I had my first one-on-one conversation with a history academic there in my Honours year. I could have done better in this regard, if I had been willing to put myself out there a bit more, but it's not an unusual experience. With regards to your second major... languages are always useful, and tend to open doors to new sources and literature that are very valuable once you hit the research end of your studies. Options like psychology, anthropology, sociology etc also open methodological doors. I'd advise taking some introductory courses and seeing what you like, rather than planning this out ahead - you've got freedom in your first year to try out a lot of different stuff.

With regards to your choice of specialisation, it matters most that you like what you're studying - a PhD is hell if you can't maintain enthusiasm for your specific subject area. But... ancient history is not a growth field (even by the standards of Australian Humanities research, which is broadly not doing great). I'd consider using university study as a way to reassess your interests and trajectory - take courses across time periods, try out new topics and perspectives. History at university is a different experience than at school, and you may find yourself enjoying different things than you expect. Keep an open mind, especially with regards to subjects which are more vibrant in Australia (non-Western histories, Indigenous history, imperial/global history stand out in my mind as areas where Australian academia is doing well at the moment). Your other option is moving overseas, which is what I did, but that's obviously a big step, and easier said than done depending on your circumstances.

I'd also note that one of the huge strengths of Sydney as an institution is student life. If you're dedicated to making it as an academic, it might seem pointless and distracting to look outside your studies for ways to spend your time. I'd suggest this is the wrong attitude to take - soft skills (event organisation, administration, public speaking...) are as important in an academic career as in any other. Chances are, unless Australian politics changes dramatically in the meantime, you'll be studying alongside a future PM on campus. Getting to know the people around you is - both cynically and not - a good idea.

Hi there, and good luck at USYD! Its reputation for history is well deserved and I hope it'll serve you well. Some basic pointers, which will likely cover some things you're already aware of if you've been ruminating on this for a while. For context, I'm an ANU grad - I'm making some assumptions about USYD's pastoral support.

1. It's a long way off of course, but you're right that your second major, and your honours field, will matter. If you're aiming for research track to teach / research in Australia, it's very common for your pathway to go BA --> HONS --> PhD, eschewing a Masters.
2. The best thing you can do will be to book an appointment with a careers advisor at USYD once you're enrolled. Your first year - or at the very least your first semester - will almost certainly consist of 1000 level courses which are often fairly interchangeable and will cover the bases for your likely majors. There'll be plenty of time to sort out your degree structure between now and S2 2020, and the best thing you can do in the meantime will be to speak with professional pastoral staff at USYD. With that in mind:
3. If you're aiming for an eventual doctorate and lecturing in ancient history, languages will be important. They may well already figure into an Ancient History major at USYD, but if they don't it will be well worth your while scoping out modern or ancient languages relevant to your interests. I'm not qualified to speak on the field which should interest you nor the job market for professors, but as you've noted it won't be good basically no matter what. Where you may see more openings in the coming decades is in non-European Ancient History, particularly in Australia. If you're interested in Asian ancient history, it remains breathtakingly understudied and under-reported in western academia, which comes with both opportunities for work and the necessity to speak the languages of relevant academic discourse. With that in mind:
4. Your degree structure should have space for some electives even with a double major, but a potential spanner in the works there will be if you spend six months on exchange somewhere down the line. If that sounds like it has appeal, then you will definitely want to speak with a careers advisor and begin plotting course timetables out in advance. It can be notoriously frustrating to get League of Eight unis to recognise courses taken on exchange as being valid for majors / minors.

Finally, and this goes without saying, but keep an open mind. It's worth planning ahead, of course, but there is only so much you can functionally do to plan ahead for a job market that may not exist or may look completely different in 8-10 years. You will be best off planning out your BA with a course advisor, or speaking with your course tutors early next year, about potential pathways and opportunities. Those tutors will almost certainly be postgrad students on the track you're thinking about now, and their advice will be very, very valuable to you.

Plus making friends with your tutor is always a good idea as long as you're not being weird about it.

I hope this helps!