You'll need to enter a US based address at the checkout. All codes activate on Steam. (These games are also on Xbox Game Pass)
Guardians of the Galaxy Standard Edition, $6 USD
Guardians of the Galaxy Deluxe Edition, $7 USD
Avengers Standard Edition, $4 USD
Avengers Endgame Edition, $6 USD
Supply you with a ROW key so works in Aus fine
Just grabbed Avengers and guardians of galaxy deluxe myself and installing as I type