"With such small restrictions on luggage size; what to pack?"
What are the luggage restrictions? When I took this trip last October we were allowed one large (up to 62" total) suitcase limited to 50 pounds and a carryon limited to 15 pounds. They do weigh the carryons so be careful. Has this changed?
My 25" suitcase weighed about 44 pounds and I had clothes that I never wore. We had to handle our luggage so much that I wish I'd taken less. Some people did very well with less than I had. I would guess 22- and 24-inch suitcases.
The handbook has a suggested packing list which is a good guide.
May will be cooler than Oct but I think layering is still the key. One warm jacket and a lightweight rain jacket, two pairs of comfortable walking shoes. Wear the heaviest shoes and clothing on the flight. One sweater (cashmere is warm and lightweight) and three pairs of comfortable long pants; socks, hat and gloves for N.Z.. Long sleeved T's or shirts and a camisole or some type of undershirt (silk?) for layering when it's cool. Bring a scarf for your neck in N.Z. A bathing suit for Fiji.
No one on our trip ever dressed up; women wore capris or long pants, no skirts. New Zealand's south island will be the coldest but you'll only be there a couple of days. There are great shopping ops there for beautiful sweaters, warm jackets, hats, etc. and yarn if you knit or crochet.
Go as light as you can and save space and weight for something wonderful from N.Z. Most important , at least o me, -- socks and shoes to keep my feet warm and comfortable.
I've traveled with OAT too and kind of like the larger gct groups, although I'd prefer less than a full bus. You get to know people within a couple of days and will meet other singles or probably female friends traveling together.
Don't fret too much about packing, it's a great trip and you'll have a wonderful time.
One more thing: if you bring food of any kind into Australia be sure to declare it. They will ask what you have and may want to see it. I had packages of peanut crackers and they ok'd that. Get rid of any fruit on the plane. Buy Tim Tams in Australia.