If you are going to pay, you need to pay soon. Indexation is calculated and applied to you help debt on 1 June so if you pay it off before then, you avoid the indexation for the year.
You won't get a better tax outcome since HELP/HECS debt is after tax money BUT paying it off may affect your return. Your employer (assuming you are employed here) may be withholding higher amounts of PAYG-W as you have a help debt. Where this is the case and you have paid the debt off already - you'll get a higher refund.
Depending on the amount of debt you have, it may or may not make sense - but thats more of a finance / financial advice question - so it really will depend on your personal circumstances. That said, paying it off will increase the amount you can borrow.
Why does it depend?
Say you have a 40k hecs debt and 40k cash on hand. You can pay it off with after tax money or use that money and invest it (or hold it in the offset account), you would, on average, make more than the indexation factor. Since the HELP/HECS loan compounds at indexation but your investment would compound at a higher rate, over time, not paying it off would more "worth it" (assuming you are smartly deploying the capital) and not blowing it. Also no-one would ever lend you money at the indexation rate - hence the saying that it's the cheapest loan you're going to get.