Obviously depends on what you want to do, and which countries you're most interested in......I spent 2 weeks in the US, 9 months in NZ, one week in Oz and then a week in Hong Kong. I was most interested in NZ, though had planned to spend about a month in Australia but by the time I got there I wasn't very interested in it. So my advice is - prioritise, and be flexible!
Within your 6 months, I'd probably spend about 5 weeks in NZ minimum if you're wanting to do all the 'usual' sights. 2 weeks on the North Island and then 3 on the South. That's a decent amount of time to get around if you're only there for a short time, see the highlights and not be too rushed.
Fiji....I'd say 2 weeks or less. Depends what you want to do of course, whether you just want to head out to the islands and find a beach to relax on, or explore the larger main islands visiting towns, rainforest etc.
Australia is obviously huge, so unless you make that your absolute priority and spend most of your time there, you won't see too much - probably the well trodden path of the East Coast (incl. Sydney) & Melbourne.