Yeah has to be Australian.
There are a lot of pieces by composers like Carl Vine out there but liking and enjoying playing them will depend strongly on your music preference.
E.g. My teacher recommended Carl Vine Sonata No. 2 (there are recordings on YouTube). I saw it and it had heaps of accidentals (which I like as modulations in Chopin, Liszt, Tchaikovsky - romantic composers) but this imo is really something else. I don't like it at all, which if you feel the same way you can probably see why (no logical patterns, dissonance, not nice melodic material and no clear distinction between musical roles - again this is all opinionative). If you're into this stuff, I know quite a few pieces you might want to play but I'll move on, you can PM me for more suggestions like these.
There are two main pieces/composer I highly recommend (which are very popular choices):
1. Theme and 12 Deviations - Sonny Chua
Great piece due to its contrast in styles and you can pick which ones you like the best for your performance (to adhere to the 5 min limit of HSC performance)
2. Elena Kats Chenin (Rags)
Particularly, Russian Rag, she has really good rags that fit for HSC criteria.
These are popular choices but imo sound amazing. Check them out and let me know what you think