Why is it not, "j'ai mon billet a l'australie" (no accents sur mon ordinateur)
First, because à l'Australie is always impossible. Australie is feminine, and we generally use the preposition en before feminine countries (en Australie, en France, en Italie...). Then, when we speak about a ticket to a destination, we always use the proposition pour (un billet pour Paris/l'Australie/l'Asie...). It's because since in French, we use the same preposition for movement and position (en Australie means both in Australia and to Australia), we have to make it clear that the ticket is to Australia, and not in Australia.
I feel like it's a bit weird to say "mon billet" instead of "le billet" or "un billet" in French.