To be a dentist, you have to do the GAMSAT - it's a postgraduate exam and then you have to undergo a rigorous interview process. You require high marks throughout undergrad at uni to even qualify for the interview though (and not to mention GAMSAT).
GAMSAT tests science knowledge of chemistry, physics and biology (I believe at a first year uni level -but dont quote me on this). It also tests English and critical thinking.
The best preparation for GAMSAT would be a Bachelor of Arts OR a Bachelor of Science degree - with a major in like chemistry and a minor in physics, whilst doing electives from the biology stream to get a 1st year knowledge. You should also do electives from the philosophy department to prepare you for the critical thinking section of the GAMSAT.
Following the GAMSAT (and 3 hard years of undergrad study in your BA or BSci), and assuming you qualify for the interview process and get an offer - you can study dentistry. Dentistry programs are typically 4 years long and are called a Doctor of Dental Medicine (DMD). These are offered by USyd, UMelb, UQld and UWA (and perhaps, possibly a few other universities).
After studying a DMD for 4 years - you are a qualified dentist, and on average would make $80k straight out of uni. If you want to be a specialist (e.g. maxio-facial surgeon or orthodontist etc.) then you have to work for 2 years after recieving your DMD and then you qualify for a Doctor of Clinical Dentistry (<insert specialisation here>). These degrees are 3 years long and cost about $35k per year, see here for further details:
http://sydney.edu.au/courses/Doctor-of-Clinical-Dentistry-Orthodontics (this is the orthodontics specialisation - there are other specialisations like oral surgery, which will allow you to become a maxio-facial surgeons. So in essence, to be an orthodontist, you'd need stellar grades and your business card would look something like this:
B.A. (Chemistry) (Syd)
D.M.D. (Syd)
DClinDent (Orthodontics) (Syd)
From what I've heard the BA (Chem) is the best prep because its flexible and allows you to study biology and physics and also do some philosophy units which will help with the critical thinking section of GAMSAT. It has no core units in mathematics like the BSci so you get to do what you enjoy and the BA (Chem) is ALOT more flexible than the BMedSci degree and tends to be easier so you can get a higher WAM/GPA and do units that prepare you well for GAMSAT. Don't do the Bachelor of Oral health, that is the worst preparation ever for the GAMSAT and will NOT help you be a dentist.
So in total:
3 years undergrad + 4 years DMD + 2 years work experience + 3 years DClinDent. = 12 years in total.
Alternatively, if your willing to travel interstate and live out of Sydney - there are undergraduate dentistry programs available.
A friend of mine got into La Trobe Dentistry and the program is 5 years long and looks something like this:
Bachelor of Health Science (Dentistry) - 3 years
Master of Dentistry - 2 years
After this at La Trobe, you are a qualified dentist and then following 2 years of work experience if you study the DClinDent at USyd/UMelb/UWA etc. you will be a qualified specialist (maxio-facial surgeon or orthodontist)
See here for further details:
http://www.latrobe.edu.au/courses/dentistry
Similar undergraduate dentistry programs are offered by University of Adelaide (which offers a 5 year Bachelor of Dental Surgery), Griffith University and University of Queensland. Along with La Trobe, these, to my knowledge are the
only current undergraduate dentistry programs offered in Australia.
The University of Western Australia had a Bachelor of Dental Surgery as well, but is phasing it out this year and replacing it with a Doctor of Dental Medicine - which it seems is more renown and is the dental degree of the future.
Hope this helps!