Firstly, try a CC Chargeback. Probably easier and faster.
If that fails, for a consumer complaint it doesn’t matter where you are, only where the business is.
You should find out which state or territory they are in.
Then:
document you attempted calm resolution and they refused. Usually this means either writing down the details of a phone call or saving an email exchange.
after making a reasonable effort to resolve, file a complaint (free) with the state’s equivalent to the Department of Fair Trading. The forms are normally online. List of agencies by state here - https://www.accc.gov.au/contact-us/other-helpful-agencies/consumer-protection-agencies
that will normally fix things (eventually).
if it doesn’t, each state has a tribunal you can file with for a small fee. The procedures differ slightly but they will attempt to have you mediate on the phone, then a arbitrator will rule on it. You don’t need a lawyer for this - it’s very informal. You can do it by phone - in fact most of these are by phone during COVID anyway. You do need to have all the relevant documents at your fingertips and be able to refer the decision maker to the relevant page or items over the phone.
Consumer law in Oz is pretty effective. The basic Australian Consumer Law is summarised here - https://consumerlaw.gov.au
If you follow the above process, stay cool and reasonable, and be prepared to compromise, it will probably get straightened out.