Hi from NZ. Are you able to bring three options? I'd suggest:
Cool weather trio of skinny pants/interesting top/neutral cardigan or blazer
A ravishing but non-sequined dress that goes with the cardigan or blazer
A casual dress that goes with the cardigan or blazer
That way you can mix and match depending on your hosts and the events. My impression of Victoria is that Melbourne is dressy. For Brisbane, expect heat, humidity, and the beach.
I saw lots of maxi dresses being worn in Sydney last year. Antipodean colors for summer in the stores are black, white, cream, hot orange, acid green, yellow, neon pink. (I can't make this up...) Cobalt and emerald are more wearable options I'm seeing. I'm from the US living down here and it's my experience that Antipodeans are edgier overall in their style - "preppy" looks out of place and stuffy.
If you want to do some shopping while you're there Cue is good for hourglass curves and cocktail dresses if you are under 5'5', and David Lawrence if you are over 5'5". Witchery and Country Road/Trenery are high street brands of good quality for more casual wear.