Unfortunately while we have a robust system of consumer laws, many retailers are at best disingenuous about those rights.
For instance, regardless of what signs the vendor puts around the store, on the product itself or on a receipt, an item that is simply different to the item that was advertised has to be refunded. For example, a phone cover that doesn’t fit the phone it was advertised for can be returned for a refund.
If you suspect you are entitled to a refund there is a lot of information available online. One good avenue to check is the website for NSW Fair Trading.