SINGA,
1. ASA is the only ATC provider here (unless you count the airforce...who i very much doubt will be recruiting from overseas!)
2. I beleive it is still a possibility to be recruited from overseas straight into the company, they are just not "actively" recruiting anymore (i think the last exercise was a little expensive for them!). This may help :
ATC Recruitment.
3. You will have little chance of choosing where you end up unless you are asking for Brisbane or Melbourne. I'm guessing that if you were accepted and offered a job, you might be able to specify which centre you want to go to as a condition of your acceptance. Most jobs are in those 2 cities. Either way you will be in Melbourne for a while to do some training.
Good luck!