Thanks Peterfermtech, I stand corrected - the couple went missing in March 2020. According to Parks Victoria:
'Alpine National Park and Avon Wilderness Park: All deer may be hunted by stalking only in parts of the Alpine National Park and in the whole of the Avon Wilderness Park from 15 February to 15 December. The use of dogs to hunt Sambar Deer is not permitted in these parks. Hog Deer can only be hunted in these areas during April. Pest animals and other species must not be hunted in these parks.'
So, it seems the pilot was legally permitted to carry a firearm in that area at the time.