Is my travel itinerary visa-free as an Australian passport holder traveling to Thailand and Myanmar?

As an Australian passport holder, you can enter Thailand visa-free for up to 30 days when entering by air, which applies to your stay from 21/12 to 3/1 with a return to Bangkok for two nights. However, your visit to Mandalay does not require a visa for stays of less than 28 days as well. Overall, both segments of your trip should be visa-free.