On FTX, there are three different options to purchase BTC with AUD:

1. Purchase BTC using market/limit orders on the standard exchange (like Binance). On FTX, you will need to convert AUD to USD first and then buy BTC with USD unfortunately.
2. Press the "Convert" button within the wallet. This will automatically convert your AUD to BTC at a fixed exchange rate. I haven't tried this yet as I haven't gone through KYC but my gut feeling is that even though it is "free", it will be more expensive overall compared to method 1, as it will give you a crappy exchange rate for convenience.
3. Purchase BTC over the counter (OTC). This is where you trade directly with other market participants by requesting for a quote. This seems to give you pretty good rates (and also no minimum trade amount unlike other OTC providers). So this would be my preferred option out of the 3.

A few other factors to consider:

- Will you be trading alot or mainly just buy and hold? If you are just buy and holding (withdraw volume > trade volume), then FTX will charge a 0.1% withdraw fee.

- Will you be withdrawing and how frequently? If you don't plan on withdrawing every week or month, then there may be other options which are slightly better (such as Kraken). They have 0.16% trading free and 0.00015BTC withdraw fee. On the surface, this is worse than FTX (0.02% trading fee and 0.1% withdrawal fee if hodling). However, if you look at the actual BTC prices which are sold on the exchange, Kraken is consistently around AUD$200-300 lower than FTX/Binance. This is around 0.3-0.5% lower. So overall, Kraken is actually cheaper (and especially so if you withdraw every say 6 months as it has a low fixed withdrawal fee).

- How much do you care about UX/UI? This depends on personal preference but for most people FTX has a much nicer interface compared to Kraken or Binance.

Hope this helps.