In fact, you should be able to download an online tax program that'll automate it - just enter your details and it'll send it off to the ATO (Australian Taxation Office).
and the site is http://www.ato.gov.au/etax/
Thats what i've used for the last two years. hmm. should do that in the next 4 days before we emigrate :P
I've never heard of any minimum time requirements - perhaps thats to do with your not being australian ? but i doubt its the case. I've worked for less than 3 months in one place and gotten tax back from it.
ps: the system isnt too hard ...
the first 6000 you earn are tax free.
the next 14000 are at 20%
the next 35000 are at 30%
... but the program will work it all out for you.