I never said I have left the Army, I am still a current serving officer in the military, but in the Reserve. I have done 10 years in the Army, I think you have been misinformed.
There are two types of Army, namely full time and part time.
In the Full time army, you will have to pay normal tax. However, you will also receive cheaper housing and free health, & dental care.
In the Reserve Army, you do not pay tax, however, you also do not receive superannuation and your wages had been tax deducted. For example, when I was getting paid as the lowest officer rank, I was getting about $125-$135 per day, where the full time officer with the same rank was getter $56,000 before tax. http://www.defencejobs.gov.au/army/PayAndBenefits/
The abovementioned link will answer your questions.