Tas would be a good option. You don;t really need much time... 2 days is plenty... go to Hobart first off as it is a pretty city with some nice markets and boat crusies.
MEL is lovely, but you need to know where to go and what to see to enjoy it as it is not handed to you on a platter as it is in Sydney (ie harbour etc).
CNS is ok... the city itself is nothing special, some nice restaurants though... the surroundign areas are beasutiful (North and tablelands)