Australia consumer complaint letter: what should I include?

ACCC recommends setting out the problem, the outcome you want, key dates, what you have already done to resolve it, what you will do next if it remains unresolved, and a reasonable response deadline. Attach copies of supporting documents such as receipts or invoices and keep the originals. Official source: ACCC. Guide: https://getinstantlaw.com/guides/formal-dispute-letter-to-company