We’ve been to NZ four times, Fiji and AUS twice. Of those three we liked NZ the most, it’s a compact country and you can drive from the top to the bottom in a three week visit. The climate varies from subtropical in the Northland to borderline frigid on Stewart Island off the southern tip of South Island. Southeastern Australia is interesting but it a big place and more like the US than NZ.
Fiji is an OK place but it’s harder to mingle with the locals and a tad more touristy than most of the places in the Polynesian Triangle west of Tahiti. Tongans, Cook Islanders and Niueans are the most friendly of all the SP islands we’ve visited.