Australia New Zealand Fuji

Haven't been to Fuji yet, but I do love their apples.

We have looked at the Windstar 7 day Cruise through the islands and the Holland America cruise around New Zealand and into Melbourne, Australia. Take a look at those.

Air New Zealand had the best prices when we flew from LAX into Auckland and into Brisbane. View their flights and pricing. We traveled the first two weeks of September and the weather was awesome. Go to timeanddate.com to view past climate averages in all of the areas you wish to travel.

Then take a look at Lonely Planet for Australia and New Zealand - they have great itineraries. And you can read and decide what you wish to see. Is it a 3 day catamaran cruise to the Great Barrier Reef ? Or do you need to see Sydney ?

We went in early October, nice spring weather.

Scheduling your trip for Spring or Autumn in the southern hemisphere would maximize your chances of seeing more temperate weather, and it would be a bit cheaper to travel during their shoulder season as well.
Unless you really have your heart set on seeing Fiji, I'd suggest dropping it from your proposed itinerary simply to leave more time to explore the charms of Australia and NZ, which are considerable. Likewise, unless you're dedicated cruisers I'd suggest deleting the proposed cruise from the mix - the week or so you'd spend looking at the water could be put to much better use actually seeing the countries themselves.
Realize that Australia is basically the same size as the continental US. Distances between points of interest can be quite long, which means that making strategic use of flights to connect them would be essential to avoid spending all of your time in transit, especially if you'd like to see such far flung destinations as Uluru (Ayers Rock), Alice Springs, Queensland in general and the Great Barrier Reef in particular, Melbourne and the Great Ocean Road to Adelaide, and of course New South Wales and Sydney - for my money the most beautiful city in the world. A hidden gem is the state of Tasmania. If you're the adventurous types there are national parks there that will take you weeks to explore.
NZ is basically the same size as the state of California. We much prefer the South Island, especially the area around Queenstown, but if it's to be your one and only trip to this part of the world then you really should spend some time on the more tropical North Island as well.
Frankly, 4 to 6 weeks divided between Australia and New Zealand will be barely enough time to scratch the surface of those two countries.

Fiji is less satisfying than expected, a long way to go for a beach and icky food. Up to you.

Fiji is less satisfying than expected, a long way to go for a beach and icky food. Up to you.

Good scuba diving, though. Some of us will put up with a lot of compromises for that. Granted, OP doesn't appear to be in that group.

Robert has given you really good advice. Australia is BIG. Identify the places you really want to visit. Look at it as going from New York to Florida to Chicago to Los Angeles or Dallas.

This is one area I would NOT recommend a cruise. As someone else noted, you'll see lots of water.

South island of New Zealand is our favorite, although we've spent a good amount of time on the north island, too.
Feel free to PM me as husband has worked on and off for years in both countries. But it also depends upon your interests.

It's a big trip and deserves all the planning and time you have. We do like their spring and fall.

Peak season for northern Australia and the desert is our winter (May to Sep). Cyclone (hurricane) season up north is mostly Jan to March.
Peak season for southern Australia and NZ is summer (Oct to March). Winter is cool without being too cold, but still best avoided.

Spring is the best time to include all of the places you might want to see - early spring for northern Australia and the desert, late spring for southern Australia and NZ.

Sep/Oct would be my firm recommendation, with cruises from Sydney to NZ starting up in October.

Looking at 40 nights in 2018 for inspiration ...

Sep 22..Fly in to Cairns (Reef & Rainforest) - 5 nts
Sep 27..Top End (Kakadu) - 4 nts
Oct 1..Red Centre (Uluru) - 4 nts
Oct 5..Sydney - 5 nts
Oct 10..Princess Cruise - 13 nts
https://www.princess.com/find/cruiseDetails.do?voyageCode=8860
Oct 23..Bay of Islands - 3 nts
Oct 26..Queenstown - 3 nts
Oct 29..Auckland - 3 nts
Nov 1..Fly home from Auckland

These are some great suggestions. Thanks so much. I guess we missed some of the best times to visit so this trip may get pushed to next fall.

April-May (our autumn) is nearly as good, but reversing the direction of travel ...
NZ >> southern Oz >> desert >> northern Oz.
Jan- Feb-Mar is a definite no go with heat and wet.