Big 4 has been finalised basically.
There are still mid-tier and smaller firms which haven't yet advertised, and may not until the closer to the September holidays and Tambam said.
My old firm has yet to advertise (I think they'll need to look for quite a few this year unfortunately) and when my brother applied for his cadetship it wasn't until August.
You also find that after the HSC when results are released, more positions will open up in mid tier firms. You usually get a few people who exceeded their expectations or didnt meet them at all that no longer want/can undertake a cadetship.
Alternatively, you can always apply as a first year cadet next year too! A lot of the firms accept either school leaver applications or people who will have finished 1 year of uni