I have to agree with Bungo. I don't think there is a chance you'll get in as a Firefighter so I'd start exploring your other options. The 'trade' route is always a safe one to go down, so your plastering or electrician background might suffice, however, if you were to secure a qualification in either, along with your work experience, this should be fine. I too have been wanting to go to Australia for quite a few years now. My (now) wife and I went for a year in 2012 to stay with her parents in Perth (they moved out there 8 years ago). At the time I was a HGV driver but when we came back I applied to do a degree, which I completed in June this year, and I'm now doing a secondary school PGCE, so once I finish this we'll be putting the wheels in motion to finally move over, as I'll be a qualified teacher. It's been a long wait and it's nearly over but it's definitely been worth it.
Basically what I'm saying is even if it takes you a couple of years or so, it's easily achievable.