What do you enjoy doing in your free time? I believe the best job is one you enjoy. I'd quit a job in a heartbeat if I didn't enjoy it!
You could always take a gap year. I took a gap... six months when I graduated. Did the whole soul-searching, "is this really what I want to do with my life" thing. I highly recommend it, and it was one of the best decision I've ever made. Just make sure it's productive - either head off for a bit of eurotour or find full-time employment (or maybe do a bit of both).
Even when I started my degree, I did so with flexibility in mind. I thought, "Eh, if this isn't for me, I'll just transfer at the end of semester." Also, as my way of making sure I really wanted to study accounting, I did all the compulsory BComm first year units first, so I got to taste a bit of everything each discipline in BusEco had to offer. I imagine you can do the same with BSci and BArts. If you're trying to toss up between two very different paths (i.e. BSci and BArts), then why not start a double degree? You can always drop one of them after a semester or two if you realise one of them really isn't to your taste. Every time you realise that something isn't for you just brings you one step closer to finding your niche.
A lot of young adults just finishing their VCE feel they must make those "rest of my life" decisions right now. There isn't an ounce of truth to it. Obviously there are some "big" choices to be made now, but (unless you like... die next week) they aren't choices that can condemn you for the rest of your life if you make the "wrong" one. It's the whole "one door closes and another opens" thing. Life is life; just roll with it and enjoy it... there's a reason it doesn't come with an instruction manual.