I'm sorry to be the one to tell you, but you are quite wrong in thinking that your investment returns will "probably" be greater than how much you are paying on your debt. Only someone trying to sell you something would lead you to believe that a normal investment, on average, will earn that kind of return. There are periods where reasonable investments earn more than 13.7%, but then there are periods where you lose that much or more. On average, you do much worse than 13.7%. If you had said your average interest rate was 2%, I might have responded differently, but 13.7% interest is very, very high and should make you very uncomfortable. Pay it down as soon as you can.
Think of it this way, if your bank offered you a risk-free savings account right now that offered 13.7% annually (tax free!), would you use it? Yes you would, because that's far better than any risk-free asset or bank account pays. Well, paying your debt down is financially equivalent to putting money in a fantastic asset that is completely risk-free and has guaranteed returns earns far, far beyond that of any similar asset you will ever find.
You could get lucky and the next two years could be great in the market, but that would be taking a large gamble with your money. You wouldn't break even until the market hit 13.7%. Like Vegas, that gamble has negative expected return (on average the house wins).
The only exception I can think of would be making enough of a contribution to your superannuation to take advantage of the "super co-contribution" if you are eligible, or employer matching if offered. That would be a guaranteed instant return. Once you have maxed that out, I think everything else should go to your debt.
Super co-contributions help eligible people boost their retirement savings.
If you're a low or middle-income earner and make personal (after-tax) contributions to your super fund, the government also makes a contribution (called a co-contribution) up to a maximum amount of $500.
- Australian Taxation Office
TLDR: At that interest rate, you will do much better paying down your debt and then investing than you will concurrently investing and maintaining debt.