Hey AD,
September is a transitional month and not much good anywhere really. It's at the end of the season up north and before the start of the season down south. Maybe somewhere in the middle

You're pretty much screwed in Cairns for that part of the season, bugger all wind, stinkin' hot and humidity that you've never even imagined in your worst nightmare. You can't even run and jump in the ocean becasue it's full of Box Jellyfish and Irikanji. Both of these little friendlies will kill you with a half decent sting. You can wear a stinger suit, but it usually is just nasty at that time of year.
If you have January, definately come to Perth. November, December and January crank!
Sorry for the glum prognosis.
Good winds,