I agree that the 5% discount is unlikely to last much longer. And I don't trust this government one bit to not find a sneaky way to start increasing interest on the balance. Plus I hate debt. So for me, it was a high priority to pay off, but lower than other debts (just mortgage, no facepunching please). I stuck with minimum repayments until the balance was down to nearly 1 year's worth, and paid it off in a lump sum before EOFY. Thus I skipped the interest for that year, and got the 5% bonus, which was a higher return that the 6% mortgage rate I was paying.
Difference with your situation is that you can put off paying most of this debt for the forseeable future. Do you expect to earn above the threshold in Aus down the track? Do you trust that the rules will not change to take the balance of your loan out of your estate if you don't pay it off in your lifetime? Do you have other debts that are more facepunch-worthy to deal with first?
Without knowing your situation, I would prioritise other debts first, but pay off the HECS in a lump sum when you can do so with cash, and the sooner the better to maximise your chance of getting the 5%. If you have the cash lying around now in liquid assets, I'd consider waiting until near the end of your first FY back in Aus, and pay if off then to dodge the interest for that year, get your bonus 5%, but still keep your money as long as you can to work for you.