Best cities to visit in Australia and when to visit them!

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I've got some money saved up and I want an adventure. Where in Australia is good to visit and what time of year is best to go?

I went in February with Adventure Tours Australia in 2008 ,from Sydney up to Cairns , stopping at many places up the east coast, then carried on up into the Daintree Rain Forest, Great Barrier Reef , all transport provided, There were 15 of us from many different countries, we stayed in good clean Hostels & great food provided, it was a fabulous experience.