Hiya! I’m having a lot of difficulty knowing if I’m approaching the essay correctly. I’m also from Australia so there’s not many practice SAT facilities so I have no clue whether or not I am writing them really badly, and am capable of getting a good score. Are there any (relatively free or cheap) online grading sites? Or any other way I could possibly get my essays graded?
Try contacting Princeton Review online. They offer online SAT courses, so I bet they would have a way of working with you.
I tried looking at their website but weirdly it seems like they don’t have anything for Australia? But I’ll try contacting them directly - Thanks! 
Hey I’m from Australia as well so I understand your dilemma. I suggest buying the official collegeboard study guide (also known as the blue book here on CC) and using the practice tests in there. After you’ve written the essays, you can go onto the website page for book owners and comparing your essays with the sample essays (on the same topic) with the respective grades. Generally the 6 essays I’ve seen so far have been around the range of >350 words (an acquaintance of mine who scored a 2400 and 12 essay wrote 500 words)