score:5
It does appear the return leg is being treated as purchased in Hong Kong. The exchange rate used will typically be at somewhat close to the spot exchange rate for the currencies on the market and fixed by Visa or MasterCard or whichever payment processing firm your bank card uses. Typically you will get an exchange rate better than if you had exchanged the money at a foreign exchange shop so nothing to worry about there.
What should be concerning you is if there is a transaction fee.Whether there will be a transaction fee or not depends on two things. First is where the transaction is executed by the airline, in your home country or in the foreign country. In this particular case the chances are highly likely it's being executed in Hong Kong and thus in Hong Kong currency so there is a foreign transaction.
The second issue is the kind of debit card you have because the percentage of foreign transaction charges you will pay depends on your bank card terms and conditions.Some debit cards waive any foreign transaction fees in which case you're home free. Most charge from 3% to 5% foreign transaction fees. If your card doesn't waive foreign transaction fees, you will be on the hook for that plus the nominal loss from the currency exchange.
Upvote:5
When I go to buy my ticket in qantas (HK -> New Zealand), I found out that I can only buy them in Hong Kong dollar.
You can buy air tickets in any currency, either by visiting a local travel agent, sometimes by using a website located in your preferred country, or in some cases calling the airline and requesting the price in the alternative currency. The website solution is usually easiest.
For instance, expedia.co.nz will always price fares in New Zealand dollars.
In these cases, the conversion will be done from the published fare to your preferred currency at the IATA Consolidated Exchange Rate. Between major currencies, it is essentially an average of yesterday's ask rate, taken from OANDA, updated at 0430 MontrΓ©al time.
If you choose to buy in HKD using your debit card, then a conversion will be done by your bank automatically into NZD. It will happen at whatever rate they determine, which may drift from the spot rate because of rate loading and transaction fees. Sheik Paul's answer has more detail.