I'm not sure what the rules are in Australia, but I know in the States in order to be able to fight in, what I'd assume any sanctioned amateur event, you must be 18 years of age or older.
Also, I have yet to train for MMA, but I'd imagine that you'd have to train both stand up and ground more than just once a week. The place where I'm planning to train has beginner classes 2 times a week and has fighter practice once a week and an open gym once a week. I'll be there at least 2 times a week while I'll most likely be there 4 times a week.
I'd work on conditioning and if you can fight in Australia before age 18, then I'd suggest finding a place that you like and would like to train at and just go at it as much as you can.