The company bills you in AUD but from a billing entity overseas. This might be like the situation where you choose in which currency to be billed, I'm not sure on the exact mechanics. But close enough.
Crucially, you are not informed that you are being billed from an overseas entity. No indication whatsoever.
Your bank, seeing a charge from an overseas entity, applies their surcharges. Whatever they are.
And so you pay the amount you expected to pay - totally cool - plus some surprise surcharge - totally uncool.
Has some additional information. If the merchant-ID for an online sale is outside the US, a US credit card company might charge a foreign transaction fee even if the sale is in USD. (This isn’t a currency conversion fee.) Substitute AU for US above as needed for Australian customers.
Personally it's somewhat unlikely that I would have encountered it, because I pay in the currency of the seller whenever possible and let my bank do the conversion to EUR (their rates seem generally better what I have seen elsewhere).
Consumer organizations generally advise not to accept those conversions offered at the seller end. But the reason being the rate, I have never heard the reason to avoid an overseas billing fee. That sounds like an American/Australian/who else "invention".
Smart move. You can be sure that when a bank offers you convenience, it's not to your advantage.
The absolute worst are ATMs, where a "guaranteed exchange rate" can cost you 50 - 100 Euros, depending on the amount withdrawn.
If you are billed in your domestic currency the location should not be relevant to my understanding. But I have little exposure to how things work in Australia.
Edit: It has never happened to me that I was billed in a different currency than what was displayed a checkout. However, sometimes there is a little checkbox preselected that they would do the conversion into the currency of my card. I always unselect that. Haven't seen a single case where the rate of my bank would not have been better.