Don't worry - no matter how badly you do in your SACs, you can turn it around in your exam. I did year 12 last year, and my two favourite examples of my study scores would have to be music and methods.
In music, I got full marks for ALL of my SACs, minus one. I had an easy A+ for GA1 and GA2 - but I fucked up in the exam, and I ended up with a study score of 25.
In methods, though, I had trouble in a few SACs, and didn't get as high as I would've liked. In fact, when the awards came round for top of classes, my school assumed I wouldn't do that well and three other students got the top awards. Then, I did brilliant in the exam, and got 38 - second highest study score in the whole school.
Honestly, I wouldn't worry about a lousy start - my start looked about the same as your's, and then I took a 3 week holiday in the middle of the year, and still walked out with an ATAR of 93. As long as you don't drop your marks (even if you do, don't stress too much) and do well on the end of year exam, you've got nothing to worry about.