Here is the history of amazing Race visits to Australia:
2-Sydney, Adelaide, Coober Pedy
4-Brisbane, Mooloolaba, Cairns,
9-Perth, Freemantle, Darwin
For Amazing Race Asia, ARA1 visited Sydney
Coutzy is right. Malbourne is the only really large Australian city not visited yet (I don't put Canberra or Hobart in that category). It should be next when they visit Australia and I think Hobart would be appropriate also.
Very much correct about Canberra. There is a fair bit to do, but the city itself is tiny.
I'd like to see more of Adelaide as well, instead of just the snippet we got between Sydney and Coober Pedy.
Given that Melbourne is considered (By locals at least) the sporting capital of Australia, and that the biggest football (AFL) stadium in the country is situated there (Along with loads of other great stuff, including wine in the Yarra valley. Sorrento and the Mornington Penninsula (More wine!) looks marvellous. We also have a good historical backing with the gold rush of the 1850's in Ballarat and Sovereign Hill.)
A potential leg could look something like this:
Fly into Melbourne
Fast Forward: Participate in a game of Ninepin at Sovereign Hill until a certain score is achieved. Fly by helicopter directly to the Pit Stop (Since Ballarat is a long way from Melbourne itself, and without the fast way to the Pit Stop, the FF is no real advantage.)
Detour: Kick x amount of goals at the MCG using an Australian Rules football, or win x amount of hands at blackjack at Crown Casino.
Additional Task: Ride the Scenic Railway at Luna Park (The worlds oldest continuously operating Roller Coaster), in St. Kilda (While there, play a game of "Spot the flimsy pretext we used to be able to put in a U-Turn") Anybody who has been to Melbourne and seen Luna Park would understand how iconic the place is.
Roadblock: Stomp on grapes at a vineyard in the wine district of the Mornington Penninsula to make x amount of bottles of wine.
Pit Stop: The entrance to Gumbuya Park, near Pakenham (Giant Peacock out the front, quite the memorable destination)
Oh, and self driving throughout the leg, because it's always fun to watch Americans try to drive on the left!