The carry trade is borrowing in a cheap yields, and investing in something higher yield. You can get very creative with it. Currency pairs, bond pairs, currency/stock etc.
Back in the 2014 EU Crisis, you had short German Bonds, Long PIG bonds, and when those heavily in-debted PIGs had issues, you saw the unwind of the carry trade and a crisis. In the Asian Financial Crisis, you had many folks borrowing USD, investing in ASEAN currencies and bond that had higher rates and a unsustainable dollar peg, which also blew up. Carry trade can also help explain forex moves, for example the high rates on USD right now attract investors in low interest rate countries like JPY, CNY, CHF to sell their currencies and buy USD.
In Japan you have Short Yen, Long Something Higher Yield. The carry trade in short yen is enormous right now given their decade of ZIRP, as shown by the slight move in exchange rates back in August leading to some catastrophic rolling margin liquidations.
In our particular case, the futures contract is Short Yen / Long Aud. Then OP is using Aud to acquire Japanese Stocks, so the trade ends up Short Yen / Long Japanese Stocks. The future contracts being necessary here since he can't go short Yen in his IBKR Aus account directly.
I think you have the margin backwards for future contracts. You only put up a small margin amount for the contract value. Initial margin on Yen/USD futures is 3k on 12mm Yen Contract (80k USD). Put in another way, you have 3k USD tied up to cover a 12mm yen position.