What a lovely school/father you have.
A)There is no such thing as "failing" the hsc because nothing is given as a pass/fail mark etc.
As for options (I assuming this is in case you don't get into uni right off the bat):
1) Get a full time job somewhere and earn money. If you not studying, at least make some money. There are quite a few entry level jobs out there for HS leavers if you wanted to work. I know of quite a few people that started working full time out of school, and now they work for companies that allow them to study (part time in the evenings) to help them with work. Look on sites like seek using key words like "school leaver" and "entry level". When I finished school my atar wasn't good enough to get into the course I wanted. I ended up applying for a bunch of different jobs around the city for admin/receptionist work. In the end I was having a phone interview and they asked why I wasn't going to uni (I -could- have gone to uni, just not my preferred unis), and they offered me a different position as an accounting cadet. Long story story I'm now studying accounting at the one uni I didn't want to go, but I got a pretty good career when I wasn't looking. And the girl who got the admin job is now my friend
2) Study at TAFE. IMO; TAFE is for practical skills/jobs and University is for white collar workers. Many trades do make a great amount of money if you work hard. Or you could just study something that interests you and then apply for TAFE next year (or as a mature student when your 21). My friend got a ~55ish ATAR, he did TAFE for a year and worked hard, and is now studying a ~95ATAR course at UTS
3)Travel! Your never going to have a better chance in your life to just pack up and see the world. You've got no debts, no commitments and it gives you plenty of new experiences! I get that a lot of parents don't approve of this sort of thing, but you never know what you might become. Maybe you'll even find a job and end up living overseas? Haha who knows!
4) Don't fail. Not really a backup plan but if your determined to get to uni straight out of school than start working harder than ever now!