As a recently graduated dentist with a partner that is a geo I think I am qualified to answer.
Dentistry study is hard, work is frustrating and so is dealing with the general public, but personally rewarding. I love helping people, and truly think I am a medical doctor, specializing in mouths (notice your local GP will treat your whole body but stop at your mouth!). Don't forget the additional health risk (sharps injuries, postural injuries, and stress...)Don't think financially rewarding like specialist. Its ok, you will be paid about the same in the mining industries as well. Check the employment ads online for an idea od the range of wages you get paid as a public dentist. In private industry, unless you own a practice, or you are extremely busy, pay is a little over average. The standard rate of pay in private industry is 40% commission of what you bill, then subtract your super (-9%) then subtract your lab fees (about $300 per crown for eg.). And then take out the tax. Some days your DA may be paid more. Some practices will pay you a retainer or wage, but be careful you are not signing up for a contract that will keep you stuck at a practice for years where you aren't happy. And in the next couple of years, Australia will have an oversupply of dentists like never before. It will be still easy to find work, but you will have to accept that to find full time work, you won't be working in the city first year out. Think maybe Moranbah- wait, that's a mining town! hmmm....Yes, dentists do fly-in, flight out work too. Oh as a female dentist, you will be always harassed about when you are going to have your first kid, and how great dentistry is for a female because of the flexibilty of working part time. Even if you have no intention on having kids. And employees in private practice, like it or not, will prefer young males if it is a full time position, who are less likely to go on kiddy rearing leave.
Geotechnical engineering is also a broad field. You may be checking mine shafts for slips and structural stability, to machinery, (to testing soil on the field). You may also gain employment at universities, CSIRO and other smaller technical companies. If you actually want to do more real soil testing stuff you maybe should think of environmental engineering (that also are employed by mines in great numbers). As a female, you will actually be more desirable, and mining companies aim for equal opportunities and females are often placed in positions of higher responsibility. And they are not all butch hard-yakka girls, a lot are just everyday people. Not all mining work is on the field/ fly-in also. Most big companies eg. Origin, BHP etc bases are in big cities, and if you have a more technical job, ie analysis data that comes back from mines, you may just have an office job like an IT guy. Occasionally expect to go on the field (2 or three times a year), but that's reasonable. Downside with these jobs is that you will probably have to stay at uni just as long as dentistry, as most employers like to see Honours next to your name, or better yet a doctorate or phD. It can get repetitive, and frustrating in different ways. Upside it that a lot of big companies will employ you first, and then pay you while you do postgraduate studies.
Experience is good. Its a bit hard for geo-eng unless you've got contacts, or chat with academics at unis, they are secretly very passionate about what they do and will bend over backwards to help a keen student. As for dentistry, go do some part-time dental assisting work, that's what I did. I also watched some other health specialties and decided against them, but its a very personal thing.