It stays in Australia, they retain the Tom Richards trophy.
Naturally it stays with the current holders of the cup (Wallabies) Just like if all the Bledisloe cup games this year end up as drawn matches -- the current holders (All Blacks ) retain the cup. Another scenario to consider is what if the Wallabies win on saturday night and the 3rd test ends in a draw after 80 mins ? As far as I know, there is no rules saying that both teams must play extra time until a winner emerges.
I wouldn't worry about that if I was you.
