Hiya
Im also travelling to Iran with AirAsia soon and Im also Australian. On the Airasia website when you look at their visa and passport page you can put in your flight and passport details and when I do that for my trip to Iran it comes up that you need a visa and that VOA is available for eligible passport holders. This is what it said exactly:
"Visa or additional documentation is required for Iran.
Visa can be obtained on arrival for visitors holding normal passports and arriving for tourist purposes only. Passengers must be arriving at Imam Khomeini (IKA), Mehrabad (THR), Mashhad (MHD), Shiraz (SYZ) or Tabriz (TBZ) International airports.
Residents of Iran can enter with travel documents that are valid on arrival.
Visa is required for passengers who work as camera crew, or whose work involves either journalism, reporting, or photography."
Hope that helps