The first time my friend and I went to Australia and New Zealand was in late October, not on any tour. We picked that time because we had been invited to a reception at the University of Melbourne and worked around that. October equals March in the temperate zones of the Northern Hemisphere. Sydney can be quite warm and Queenstown will be much cooler. We swam in Cairns in the hotel pool, and a lot of people swam at the Reef.
If you pick a basic color for your slacks and then tops that will coordinate with them, it will be fine. Sometimes, I choose black and that can include navy and gray. Or medium brown to taupe. Solid colors for the pants and then the tops can be as colorful as you want.
We managed to use laundry facilities, but we were moving pretty much at our own pace, and most of our hotels had coin-operated machines. I always rinse out my undies on any trip.
I've never been to Fiji, but from what I've heard the clothes for there will be much lighter and won't take up much room.
And if you have a Kindle or I-Pad, you won't have to make room for books, which was always my major problem.