Australia travel insurance claim denied: what should I do first?

Ask the insurer for the written reasons for denial and the exact policy clause or exclusion relied on. Keep the policy wording, claim form, medical or travel evidence, receipts, cancellation records and correspondence. If the internal complaint process does not resolve it, AFCA can consider many travel-insurance disputes. Official sources: AFCA and Smartraveller. Guide: https://getinstantlaw.com/travel-insurance-claims-disputes