Sydney and Melbourne are chilly in September. The Gold Coast is a good option, Byron Bay, Brisbane too. Cairns has an excellent climate all year round and diving/snorkeling in the Great Barrier Reef is once in a lifetime.
For resorts look into Fraser Island perhaps. I loved it there!
I went to Melbourne for 6 weeks. I did not stay somewhere all inclusive, but it was amazing there. I went to Sydney for a weekend of my trip and did not enjoy it as much. Australia is huge though, where exactly are you looking to go?