Under ACL you have the right in regards to a major fault to repair, replacement or refund for a major fault (not sure the Media Head unit is a major fault). If a minor fault it can be repaired, replaced or even refunded or compensated at the supplier’s choice not the purchaser’s. However in your case the use of Refurbished Parts does have some coverage under ACL:
" Refurbished goods or parts
A refurbished good or part is likely to be considered as a used good/part that has been reconditioned or restored to an acceptable working order.
If a repairer sometimes uses refurbished parts to fix defective goods rather than new parts, or sometimes replaces defective goods with a refurbished version, they must also give the consumer a repair notice before accepting goods for repair.
Unlike the notice for goods capable of storing user-generated data, the notice must include specific wording required by the ACL.
This wording is:
Goods presented for repair may be replaced by refurbished goods of the same type rather than being repaired. Refurbished parts may be used to repair the goods.
A repairer must provide this repair notice whether or not they know before inspecting the goods that they will use refurbished parts in a particular repair or supply refurbished goods instead of repairing the goods.
A repair notice can be included in another document provided by the repairer (for example in the terms and conditions document for the repair) so long as it clearly distinguishes the notice from other information in the document, stating that the notice is given under the ACL, and is not hidden or otherwise obscured within the document (for example, in fine print) and provided before the repairer accepts the goods for repair."
I’m not sure if the Warranty document is considered such a required notice in regards to the repairs you had undertaken or if a particular notice had to be generated for this particular repair. This is the only thing that would require clarification, if the warranty document is considered the notice you may not have a complaint you could raise with them, but you could still supply them with feedback about the issue and there is nothing to preclude you from asking if they would consider some compensation. If the Warranty document is not such a required notice and you before the repair did not receive the required notice you can make a complaint to your Fair Trading department and perhaps seek some compensation/redress in your Civil & Administrative Appeals Tribunal. You could seek some free legal advice to help determine the merits. As I am not a legal professional the advice I give is only my thoughts and should not be relied upon.