3-year-degree from Aust doesn't cut it. You need a 4-year-degree. If you can do the honours at your university, then do it. But I guess it depends on your GPA.
Military Administration will decide based on your health.
Army: 21months
Navy: 23 months
Air Force: 24 months
Civil Service (Level 4): 24 months
You can choose between Army, Navy or Air Force if you get Level 1/2/3. But it's not like you will fly anything if you go to Air Force so why bother?
So you are in 3rd year, that makes you 20 or 21. Even if you finish your national service, you will be 24 at max. Forget about getting a job in Korea at 24. It just doesn't work.
This year, KAL hired those (F-4 visa holders) who were born in 1987. You can do the maths yourself.