Being Australian myself, thought I might lend some advice. I’ve been to every State and Territory, and it really depends what you’re looking for. Not sure if you’ve been to Australia before, but here’s a basic rundown
Melbourne and Sydney are brilliant for shopping, some early history, markets, museums and the like - Sydney has the beaches, Melbourne has the trams and breathable air.
Central Australia, in particular Uluru (no, it’s not Ayers Rock) is absolutely stunning for those wishing to spend more time outdoors.
Tasmania is very lush and green with lots of wildlife, but also almost permanently raining.
Queensland is sunny and full of beaches, Brisbane is lovely and the Gold Coast has all Australia’s best theme parks
South Australia is very dry. Not so keen on it. Ditto for Western Australia.
Canberra is full of history and assorted national institutions. Has nowhere near the party atmosphere of Melbourne or Sydney, but is nice none the less.
Hope I’ve been some help