I took that trip in April a couple of years ago. I didn't see people wearing shorts anywhere, even in Fiji, but did see capris, probably in Cairns and in Fiji, which were the warmest on the main trip. Some women on our trip who'd taken the Outback extension wore capris a few times.
Other than those two sites, I don't think October would be warm enough for shorts (60s-70s?) and I wouldn't wear them in Sydney any time unless you're going to the beach. It was cool and rainy in S. when we were there.
You'll be fine layering in NZ with the clothes that you plan. The checked limit is 50 lbs, which wasn't difficult for me. The carryon limit is 15 lbs and Qantas does spot check and weigh the bags. The handbook advises against wheeled carryons but my 19-inch was fine; it always fit in the overheads and there was enough space in the luggage storage on the buses, it wasn't any larger than some of the non-wheeled carryons.
Be careful of the food limitations, Australia and NZ customs take it seriously. If you're bringing in any food, tell them and they may or may not let you keep it.
You're in for a wonderful trip. I'd go back in a heartbeat if it wasn't so far away.