You've already lots of great advice. I just have a few comments.
If you decide on Tasmania, make time for Maria Island, a half hour by ferry from Triabunna on Tasmania's southeast coast. Triabunna is a 1 hour 15 minute drive north of Hobart in the direction of Freycinet National Park. In fact, you can see the Freycinet Peninsula from Maria Island.
Maria Island is a wildlife haven. Wombats, Cape Barren goose, Forester kangaroos, Tasmanian pademelons, Bennett's wallabies, ringtail possums, and Tasmanian native hens are easily seen. If you spend the night, your chances are good of seeing a Tasmanian devil. In addition to wildlife, the island has deserted beaches and bays, walking tracks, geological and historical sites. including "the most intact example of a convict probation station in Australia, and one of eleven convict sites recognized by UNESCO as a World Heritage Site".
Maria Island doesn't have motels, hotels, shops or restaurants, so if you'd like to spend the night, you'd have to camp or stay in what used to be the penitentiary, but is now basic accommodation. You'd need to bring a sleeping bag, food and a flashlight/torch or candles, as there is no electricity in the rooms. However, there is electricity in the mess hall. A lot of day-trippers visit Maria Island, but when the last ferry leaves, it's a quiet place, great for wildlife viewing and stargazing. You can read more about it here:
https://www.parks.tas.gov.au/?base=3495
https://encountermaria.com.au/
https://eastcoasttasmania.com/operator/maria-island-penitentiary-accommodation-maria-island-tasmania/
I also wanted to add was that if you someday go to Exmouth, try to do so during whaleshark snorkeling season, which is from March to September. It was one of the most unforgettable experiences of my life. We also did a dive off Exmouth's Navy Pier, which was like an aquarium, and snorkeled with manta rays off Coral Bay, which is a 1 hour 40 minute drive south of Exmouth.
I'm not nearly as knowledgeable about Australia as Coalcliff, but I'd agree you'd need more time for those "spread out" WA attractions. I haven't been to Karrijini NP, but isn't it almost an 8 hour drive from Exmouth? And I wouldn't just go to Margaret River for just one night. It merits more time: there's the wineries, Leeuwin-Naturaliste National Park, and more (the beaches of WA are fantastic). Albany is almost a four hour drive from Margaret River. My husband and I did this drive many years ago and had planned on continuing on to Esperance, but after driving to Albany, we felt very tired of driving. Then driving back to Perth from Albany was long, too.
By the way, you can dive in Tasmania. It's temperate water diving in kelp forests, so nothing like GBR or Ningaloo, but it's interesting as well. On our dive we saw many weedy sea dragons and seahorses. You can read about Austrlaia's fabulous sea dragons here:
https://www.australiangeographic.com.au/topics/wildlife/2012/06/the-mysterious-world-of-seadragons/
Before you book your trip, check it against the school holiday schedule.
I've only vacationed in Tasmania twice and only on the East Coast, from Launceston/Tamar River Valley area down to the Hobart area and a few points in between, but it doesn't remind me too much of New Zealand (I live on the South Island). Maybe Tasmania's West Coast is more reminiscent of NZ, but I wouldn't know. I wouldn't worry about Tasmania being too much like NZ. I can think of a few differences. New Zealand has few feral wallabies and way too many common brushtail possums (both wallabies and possums were introduced from Australia and are considered pests), but apart from these, NZ has no native marsupials. I don't think there are fiords (fjords) in Tasmania, or many, if any, glaciers. I don't think there are any active volcanoes on Tasmania either. There are way more eucalyptus trees in Tasmania, though these, too, have been introduced to NZ. Almost 15 percent of NZ's population is of Maori heritage.