From what i can recall being told wether it helps or not that you need to apply for a parks assistant or caves guide job with NPWSA in order to get on their seasonal fire but im not sure that is true as i've seen jobs in the Career One section of the Advertiser where NPWSA ask for people to man their seasonal fire crews
So best advice is to touch face with your local NPWSA office in September and see if there is any vacant spots for their summer fire crew or tower crew and put your name down
As for me i'll be doing the same thing but also watching out for fire tower tenders to man the Forestry SA towers here in the South East as that is another thing that i've wanted to do