Also an Aussie here, currently doing my PhD in Swinburne Uni in Melbs; I did my undergrad at Curtin University in Perth, then tutored for a year or so before starting the PhD. So, if you have any specific q's about a science undergrad, a PhD, or even what your tutors or lab demonstrators will really think of you, let me know.
Anyway, lessee; straight off, uni is oh so very different from school. You will probably experience a fair old shock going from one to the other; you are actually treated as an adult, for starters. Professors don't care if you show up or not, as long as you get the work done (with the exception of things like laboratory sessions, attendance is not taken, or is taken only to gauge student interaction). It is entirely possible to get a HD in a subject and only show up for the midsem and final exam.
The counterpoint is, you will be expected to learn things in your OWN time. If you go to a lecture, it's to introduce a topic. Most tutes are there to help you answer any questions once you've attempted said topic at least once already. In STEM subjects like you're going for, you can expect to put aside time equal to between ~25% (1st year) to 100% (3rd/4th year) of your contact hours for personal study; expect longer if you're a slower learner than average, less if you're faster, though some units will need 100% even if you're brilliant (F*** you, Electromagnetism 302).
Also, because the lecturers expect you to be adults, they WON'T wait for you. If you struggle to learn a lesson, seek help straight away (there will always be plenty of options for this, from study groups to private meetings with the lecturers); the next lecture will be on something different. So, this isn't learning at your own pace; indeed, as you get to higher years, the course load can get pretty brutal. You will be juggling several unrelated topics at the same time and around exams, you can kiss your social life goodbye.
Learn time management though, and don't expect to be able to coast through on raw intelligence, and you'll do well.
You will find some subjects that you used to hate suddenly become exciting, because it feels less like rote learning; I despised maths in highschool, but by the time I got my BSc, calculus was my favourite subject and I had done enough optional maths that I would have qualified for a math minor, if my course had allowed them.
There will be stuff you learn that you will never use. Often, it's because it's a stepping stone to understanding how something else you use came about, othertimes it just is related to a part of your field you personally will never touch.
Of course, some things will always suck; you will grow to quickly despise statistics. I guarentee it.
Now, so far, I've probably painted a fairly lukewarm picture of uni; I won't sugarcoat it, plenty of parts do kinda suck.
BUT.
I personally feel my undergrad was the best time of my life. If I won lotto tomorrow, I would happily go back and spend the rest of my life doing bachelor's degrees. You are constantly learning new things. You are learning them from people who have chosen to devote their whole lives to the topic. The parties are pretty good too.
If you do something like engineering, you will definitely find it is a worthwhile investment financially as well; I know a civic engineer on $400k a year, which is pretty damn good for a job with no heavy lifting. That said, from what I've seen, education in general is more about providing job security than a solid paycheque; I'm going into academia, and unless I make it into the "management" equivalent (e.g. professor, head of department etc), I'm going to be stuck at around $60-90k depending on experience. While I could earn twice that going to the mines as a minimally skilled labourer, I have a job that can survive pretty much any recession; layoffs in academia are few and far between, and small if they do ever occur.
Something high demand like Eng, assuming you're working in industry, can net you a >100k income within 24 months of graduating if you find the right place, and still offer you the job security that many of the cashed up bogan crowd sorely lack.
So yes, go for it. Plus, remember, HECS allows you to at least try a semester without ever putting yourself in a serious financial situation.
Four is considered full-time in Australia? I have to take six pretty much every semester.
Most unis have full and half units; 4 full units represents a standard full load, but it could just as easily be 3 full + 2 half units, or 2 + 4.