I'd go visit Tasmania.......It's compact, you can travel between many sights in a few hours, and the scenery can be spectacular.
Maybe spend a week in Melbourne, then go down to Tassie for a couple of weeks?
It depends what you want to see..if you are a landscape person then Tassie, if you like people/cities then I'd come to Melbourne (theres also a photo market in September in Box Hill - about 1/2 hour by train from the city centre. It's the biggest photo market in Australia with around 80 tables held in the old Town Hall building)
As a born and bred Melbournian I love Melbourne, but also love wandering around Sydney, but the pace of life can be hectic compared to Melbourne..however in September Sydney will be a bit warmer, and the harbour and coastline are stunning..plus The Rocks is a great area to wander around...