Given your self-description in post #10 above, I'd say that for you, 6+ days in either Sydney or Melbourne would be too long, even including a day trip to the Blue Mountains and/or the Great Ocean Road respectively.

Australia is a great country to visit, but for me, the main attraction is the people and the culture, not the geography.

Its now time to annoy everyone else on this forum by saying that, for me, every Australian tourist attraction I have been to -- with the possible exception of the Great Barrier Reef for avid divers or snorkelers -- is about half as good as it's promoted to be. So, when in doubt, schedule less time rather than more.

However, I haven't yet been to Tasmania so this comment doesn't necessarily apply there.