I did a couple RTW tickets during my student years, amazing deals. I’d love to do that again.
As your real plan is Aus/NZ/Asia, here’s my thoughts:
AUS: massive country but you don’t need to see all of it to enjoy it. Sydney, and surroundings, is amazing, expensive to live, but if you’re solo and do the hostel thing, manageable. Melbourne equally as interesting. Tasmania, gorgeous. Cairns and Great Barrier Reef, stunning. Those would be my must dos. Anything else is bonus. Interior flights are relatively cheap. Cairns will be the most expensive. On a tourist visa you can spend 3 months and easily see quite a bit in that time. Note, you can only really enjoy the GBR in the winter time in Australia, which is not generally the best time to visit the other cities, but not the worst. Australia doesn’t get freezing winters, so you can enjoy the cities and avoid the crowds.
NZ: absolutely beautiful place. North Island and South Island are completely different. North Island is fine to visit, few places will blow you away and the major cities, Wellington and Auckland, you could spend a week or two in each and see everything you’d want to (a week each would be fine). South Island is another story. Spend a month here at least and take your time and explore the whole Island, it’s so stunningly beautiful and diverse, you won’t believe your eyes. You can just rent a car and drive everywhere and stay in hotels or Airbnb’s. There are thermal hot springs, stunning mountain towns, glaciers, wine country, Queenstown and Dunedin. Honestly, I spend 2 months and just soak in the beauty and tranquility. Go from Dec to March timeframe, when it’s the warmest
Asia: wow, massive area. I’d go to China, Thailand, and Bali definitely. Vietnam is meant to be amazing. Malaysia too. They’re all relatively cheap to get to once you’re in the area. Check visas and entry requirements. You could spend a month in China and Thailand easily. A week or two in Bali, 2-3 weeks in Vietnam. I’d look up whenever they have their rainy seasons and try to avoid that, otherwise you should be fine.
Have an amazing time!