Marriage will get you a permanent visa, which in turn will allow you to work. Time estimates (and only such can be given) to obtain the visa after legally marrying your girlfriend may range between 3-4 months to 1-2 years. My recommendation: marry her in Brasil, get the visa in Australia via the Brasilian consulate. After getting the visa, enter Brasil with it, and apply for permanent residency. At this point, you should leave Brasil only for holidays. Police might check on you unexpectedly, and if you happen to be abroad, your residency request will enter hibernation status (yes, documents may hibernate in the tropics, and sometimes for rather more than just a few months....)
The actual work permit, however, may be obtained rather easily. Be sure to check with your prospective employer what the requirements are. If you need an ID, be prepared for another long wait until you get it...
My very personal and highly subjective advice: spend the effort necessary to set foot in the Brasilian work market to convince your girlfriend to move to Australia... I tried unsuccessfully, that's why I live here now. But I am sure glad that I am able to say to myself, that at least I tried
Boa sorte!
Pierre