Spotted Auckland to Nadi return for around NZ$730, nonstop both ways on Fiji Airways, out 15 Sep back 22 Sep. Nonstop to Fiji under about NZ$750 return is a good price for that carrier, and mid-Sept is a nice window before it gets busy.
Nonstop each way, Fiji Airways
Roughly a week on the ground, and nearby dates looked similar so there's some flex
Full service fare, so a checked bag is included
The link opens the search with the route loaded, just pop your return date in. Prices show in AUD by default, there's a currency toggle up the top for NZD.
Full disclosure, I run Sky Spy (skyspy.deals), a free flight comparison site, and that is how I found it. Happy to answer any questions.
If you are flying to fiji from auckland with a stopover you are doing it wrong