Hi Toqual
This is a more detailed response which lands in essentially the same place as JohnR's suggestions.
Regulators of the ACL encourage parties to a dispute to try to resolve their problems directly between themselves and informally before escalating the dispute. As JohnR explains, as it seems you have already tried to speak with the supplier over the phone or a visit, a complaint letter to the supplier is the natural next step.
As JohnR also points out, there are complaint letter examples available on other forum threads. Also, here is a link to the template and example complaint letters made available by the Australian Competition and Consumer Commission (the ACCC):
http://www.accc.gov.au/consumers/complaints-problems/write-a-complaint-letter
In summary, you can see that the ACCC prescribes the complaint letter should describe your problem and desired outcome, include key dates, set out action you’ve taken to resolve the problem, request a response and attach supporting documentation. It is important that you put effort into this letter, including attach copies of your receipt and screenshots of the supplier webpage, in order to communicate your case to the supplier, but also to provide an easy-to-follow factual summary for a third party if the supplier remains unresponsive.
If after the complaint letter you are still unable to fix the problem, you may need to seek assistance from a third party. These include, generally in this order of contact, your state consumer protection agency, industry ombudsmen (as applicable), the ACCC and your state small claims court or tribunal. The following ACCC webpage has simple-to-follow links to these bodies:
http://www.accc.gov.au/consumers/consumer-protection/where-to-go-for-consumer-help
If you have any further questions, please don't hesitate to ask
Hugh