There are plenty of alternate pathways into uni. Heck, even if you get a mystery mark, you can still get into university. It might take you longer but in the end, you are still in.
If you don't get into the course you want, you can always do a course with a lower atar requirement which you can get into, work your ass off, get a good GPA and then transfer a year later into the course you want to do.
Many universities have programs which you complete if you did not get a good enough atar into your course. You do it for 6-12 months and then you can apply for university is you get marks and some programs even provide guarantee entry to the course you want.
Macquarie University has SIBT and Foundation program:
www.sibt.nsw.edu.au/
http://www.foundationstudies.mq.edu.au
University of Western Sydney has UWS college:
http://www.uwscollege.edu.au
You can do the foundation program or enrol in separate units which you can use to apply for university. They also have the diploma program where you complete a diploma and you get guaranteed access to the responding degree that you want to do the following year.
University of Newcastle has Newstep:
http://www.newcastle.edu.au/students/foundation-studies/newstep/
Same thing as the foundation program.
Then there's Open University:
http://www.open.edu.au
This has absolutely no entry requirements to complete a degree at many universities including Macquarie or Monash. You don't need an atar, they don't even look at your atar. They provide online resources, and you complete the degree online. You don't go to campuses. You just study yourself at home with the resources they give you, go to write the exams and you pretty much get a degree. If you would like you study on-campus, then you can always complete a couple of units from them and then use that to apply to university. This is a really great way to get into university or to complete a degree straight away if you have the self motivation. You can even complete the degree in 1.5 years or even 10 years because you work at your own pace.
Then if all else fails, there's TAFE. People usually do a Cert IV, diploma or advanced diploma and literally every uni accepts these as an alternate pathway as long as you get good marks.
HSC is definitely not the end of the world, even with the shittiest atar you can get. The merit based system in Australia is what makes it possible!